Introduction…
The following is for the largest part based on personal experiences, complemented with experiences (and opinions) of others.
I started keeping garter snakes when I was about 13 years old (1969/1970). Fortunately my parents had no problems with it.
I went on my own to Rotterdam by train to search for a petstore that would sell reptiles (I was still naive then and thought that in a large city reptles would be sold everywhere). After having wandered the largest part of the day through Rotterdam, visiting every petstore I could find, I finally ended on the Noordsingel thanks to some tips. There was a reasonably large reptile-store, van Mourik. I did not know what I saw! All kinds of animals that I only knew from books, each more exotic than the other.
Fortunately, I was not tempted to buy all those beautiful animals, but kept to my plan: Thamnophis.
Well, he had them. Almost completely black garter snakes crawled through a terrarium; there were hundreds in it. I bought a beautiful pair and went looking for the station.
It appeared to be two Thamnophis sirtalis parietalis from Manitoba, Canada. The distinctive red flank spots were only quite evident when they had eaten. I was hooked! From that moment I kept en bred garter snakes for about fifteen years. After this period I kept all kinds of other reptiles, but took care of several Thamnophis-, Nerodia- and Natrix species, but not in my home. During the last 25 years I took care of alle kinds of reptiles and amphibians in a reptilezoo in Vlissingen (in hindsight not something to be proud of because there I saw too many examples of how you should not care for animals). But I also learned good things in that zoo and these experiences are also usable for garter snake care. At a given moment I more or less rediscovered the Thamnophis genus. Since then I only have garter snakes. Ik keep several species and subspecies.
But… enough about me.
When you are going to buy and keep gartersnakes (or other animals) for the first time, th first thing that you have to 
do is: Think, gather information and read this, think again, read more and… keep thinking!
There are many things you should be aware of if you want to buy a pet. Whether it is a snake, a chicken or a hamster. Before you start buying a terrarium and snakes you will have to ask yourself some questions to prevent some unpleasant surprises in a later stadium.
# How much time do I think I am willing to spend on my new hobby?
# What will it cost and will I be able to keep up with this over a long period?
# How much room do I have available and is it possible to expand if this is necessary when your snakes are growing larger?
# Can I get, in a simple way, the right food throughout the year?
# Is there a vet in the neighbourhood (with knowledge about reptiles) that I can visit when my snakes get ill?
# Am I willing to pay the bills of the vet, even if a new snake probably costs less?
# Is there an experienced reptile keeper in the neighbourhood that is willing to advise me when this is necessary?
# Is there someone who is willing to care for my snakes if I go on holiday?
In short. think before you act!
I will try to answer as many questions as possible to your questions.
I do not know everything and I still learn every day.
Are your experiences different than mine, please let me know. Mail me please.
Garter snakes are often considered to be snakes for beginners.
For a part this is true, but when you buy these animals when you are not or ill-prepared, things often go wrong.
When you prepare yourself, some of the species of Thamnophis are excellent snakes for the beginning terrarium keeper.
I am not going to write a complete terrarium book! Things like how to build a terrarium or what kind of thermometers there are cannot be found on this site. There are lots of books about the general terrarium keeping available and on the internet you can find also a lot of information. It is swarming with websites about how to build and maintain a terrarium, etc. and there are many forums where you also can find lots of basic information.
The info you’ll find here is mainly about things around Thamnophis, but is for the biggest part also applicable for other “fish-eating snakes”.
But now… let´s start!
Wild-caught animals?
Don´t be tempted to purchase wild-caught animals. Buy your animals in principle from a breeder.
# Firstly, there are no animals removed from nature for “your pleasure”. This is not necessarily always a bad thing, but it would be unfortunate when they come in possession of an inexperienced person who does not know what to do and the snakes are at high risk to die.
# Secondly, captive-bred snakes are, in general, not infected with illnesses and parasites and are already used to a life in a terrarium.
# Thirdly, you can get all kinds of advice from the breeder. Usually they are more than willing to tell you what the best way is to take care of things and to answer your questions. And you can also see how he/she keeps the garter snakes.
How to find a breeder?
# Contact the web masters of sites websites which are about garter snakes.
# Nowadays there are regularly fairs organized where all kinds of reptiles, etc. are sold. There often are also some stands where breeders of garter snakes try to sell their young.
# Take a look at sites like Marktplaats.nl and Ebay. Don´t you trust the offer, try to find someone experienced to ask for advice.
The actual purchase … what species do you choose?
This may depend on a number of factors. The most important and also the most obvious factor is the availability. You might think Thamnophis gigas is a great species and you want to keep it in a terrarium. But, this species is never and nowhere offered.
Or you think that Thamnophis sirtalis tetrataenia is a fine snake species to start with, but you don´t have the money for it. Or you live in the USA. For this subspecies is not cheap and the price will not fall in the near future. Next to that… it is forbidden to keep it in a terrarium in the USA.
My advise: start with a “cheap”, strong species. You’ll have a lot of fun with it and the chance of accidents and illnesses such as food refusal are fairly small. And it is allowed to make a small mistake with them. Many garter snakes namely are very durable and adapt quite easily.
These species often are easy to obtain. And As you get more experience you can always add a “expensive and/or difficult” species to your collection.
Species that are regularly offered and for not too much money are, a.o. Thamnophis sirtalis sirtalis, Thamnophis sirtalis parietalis, Thamnophis radix and Thamnophis marcianus. Also Thamnophis elegans vagrans fits in this list, but there is not yet a regular availability.
Whereto goes your attention when you purchase a snake …?
To begin with, a reliable breeder will not sell unhealthy animals. That is a matter of trust, so a good reason for you not to purchase animals from the very first person.
# A healthy snake “looks good”. Even an inexperienced person notices when a snake “doest nòt look good”.
# A healthy snake is active and alert.
# The cross section of a garter snake must look somewhat round. Than it is fed normally. If this is not the case, then the snake has lost weight for some reason. Often these thin snakes are also seriously weakened. Don´t buy it!
# The base of the tail may not be thin and triangular in shape, because this means that is very thin. And when you do not know why this snake is that thin, don´t buy the snake!
# If there are old pieces of skin present on the body of the snake (shedding the skin went wrong), don´t buy the snake. This is not normal.
# When the snake does not stick out its tongue when you touch or approach the animal, don´t buy the snake!
# When the snake has a audible breathing (I do not mean the hissing that snakes do every now and then) and/or blows bubbles and/or keeps its mouth continuously open and/or has lots of mucus in its mouth: don´t buy it! The snake has, at least, caught a cold, but also can be suffering from pneumonia.
# The cloaca of a healthy snake is clean. When you see remnants of faeces or another substance on it or around it: don´t buy that snake!
# When there are injuries, blisters or bumps present on the body, you better not buy it. The scales of a healthy snake are lying flat on the body. When they are somewhat upright, the snake might be suffering from blisters thanks to a inappropriate care.
# When you pick up a garter snake, it will oppose this and will try to flee. You can feel the strength of its muscles. Is a snake as a weak lace in your hands: do not buy!
# When you have the snake in your hands take a good look if there are small, moving black points crawling over the scales. These are snake mites and they are not easy to get rid of. Often they are well visible round the eyes. Look with a magnifier (or use your reading glasses). You better do not buy such a snake.
All the above points suggest an animal that is not healthy, poorly cared for and may even be wild caught.
Normally you will not see this kind of misery when you go to a serious breeder, but you will always have to pay good attention. If possible, ask an experienced garter snake keeper if he wants to accompany you.
When you bought an a garter snake, always ask for the name and phone number or email address of the breeder. Ask if it is ok that you contact him/her if you have any questions when you are at home. A trustworthy breeder will not have objections.
Make sure the snake is properly packed. Small snakes in a small plastic box with some small holes in it. Put some crumpled tissue paper in it and make sure there is enough in it to let the snake brace itself. This causes less stress than when you put it in an empty box. Put the small box or boxes in a lockable “curverbox”.
Bigger snakes can be placed in a cotton snake bag. Make sure it cannot get out and put the bag in a lockable “curverbox”.
Watch it! Snakes are masters of escape, so make sure that everything is well sealed.
A garter snake of 20 – 30 cm in length that escapes in the car, you will almost certainly not find again.
Make sure that it is not getting to cold or to warm during the transport. Below freezing is too cold. Best temperature is between 15 and 25 ° C. That’s basically always safe.
The most dangerous is a temperature that gets too high. When it rises above the critical value for garter snakes, the snake will die.
When the “curverbox” is standing in the sun in the car, the temperature rises rapidly to very high.
What the exact critical temperature for these snakes is, I do not know, but anything above 30 ° C is potentially dangerous. Garter snakes do not die at a temperature of 30° C., but from 30 °C to 40 °C or higher is a small step when standing in the sun.
When you are afraid the snake cools off too much during the voyage, you can place a hot/water bottle or a lemonade bottle with water of 25/30 °C in the curverbox. If the trip lasts no longer than a few hours, this is adequate. Make sure the bottle cannot roll on the snake! When the trip takes longer you have to replace the cooled water if this is possible.
You can also use a heat pack or heating pack. These are for sale in most reptile shops. Personally I have not much experience with these things.
But normally you do not have to heat the snake during the trip. Even during the winter it normally is not cold during the trip. A somewhat lower temperature keeps the animals quiet en and if they do not feel spied upon and there is enough crumpled paper in the box, the stress will be limited to a minimum.
Look at the Terrariumpage for more info about the quarantine terrarium in which you are going to place your new snake.
