Belly heat is not a requirement for any snake, the only direction snakes gain heat faster is through their backs, it's why species from cooler areas have darker coloration on their backs compared to their stomachs so they can gain heat faster from the sun. The belly heat requirement myth comes from heat pads/heat tape being really easy to use, they advise another keeper to use them, then so on so forth it becomes the only way because everyone wants to think they are right.

Water snakes do well when they aren't allowed to be perpetually wet, in fact they don't really seem to NEED a large water bowl however that aspect is kinda what makes them cool in my experience. My friend keeps his in screen top aquariums with CHE heating and a UVB hood, while rodent eating species don't need UVB there is a little bit of contradictory evidence showing that snakes that don't primarily eat mice (IE garters, green snakes, water snakes) do benefit from that UVB. These species also happen to be much more avid baskers in the wild than most rodent eating species.

How complex you want to make a setup is going to be up to your own experience level and ambition. I saw one at an aquarium that was basically a spectacular palladarium with rocks and plants it was really cool but that's gonna be outside the scope of many folks.

Feeding these guys is also a special requirement. Many people purchase whole fish, fish fillets, etc, add the necessary supplements (calcium, vitamins etc) and blend it up and create jello with it. Type in "fish jello for snakes" and go to the thamnophis forum that shows up and read up on it. These snakes (not the green snake as it's an insectivore) can be raised on mice only diets and many do however the really avid keepers seem to feel as if too many rodents will cause similar issues with these species as feeding too many rodents to say a bearded dragon in terms of liver issues from too much fat.

In terms of finding CB specimens, facebook groups dedicated to garters, water snakes and the like are going to be the place to find breeders and other advice.