Most of the smaller snakes -- colubrids like ringnecks, redbellies, and maybe the red milksnake -- are difficult to keep because their feeding habits are not well-studied. Most of the snakes you see at pet stores grow to larger sizes because most of them have to be bred in captivity. Even most garter snakes would probably outgrow an ExoTerra plastic terrarium like the one you mentioned. I think I've seen it before.
Research garters though, but their care can be time-consuming, not exactly an office pet. They need to be fed often (worms and fish mostly) and thus their cage has to be cleaned a lot.