most exotics shops will already have some form of it setup but it would not really be acceptable conditions i dont think. I cannot think of a single place ive seen terrapins for sale in worthy tanks/setups etc.
But maybe youve just found a hole in the market lol!!
but if i were you i would just stock the tank as said above with plants, if youve a nasty streak about you and you dont mind it, you could buy some lil cheap neons, i am sure your turt would enjoy them once he caught them too...but thats not for everyone's liking, i never did it but it would be an option i think