Freshwater clams at supermarkets are usually Asiatic or Manila clams (Corbicula sp.), which are the same ones sold as "golden clams" by pet stores. Clams can be tricky to keep, though. Try searching this subforum for some of the other clam threads.
I catch them in the ponds and creeks all around my house. They dont usually last very long for some reason. They dont get very big either, just dont keep them with cichlids or crayfish.
i have 2 f/w calms in my congo puffer tank which have been alive for a good few months at least and still kicking (so to speak) with no problems... i guess being filter feeders as long as there is enough micro-organisms for them to eat they will survive and considering puffers are such messy eaters i presume they are getting all the food they need from the left overs in the tank which i can't clean out???