oyster filtering capability

Question
I am doing a research project on the filtering capabilities of oysters.  I am, however, having a hard time finding a place to buy live healthy oysters. Information on where to find freshwater oysters would be highly appreciated.  If you can't find fresh water oysters, where could I find saltwater ones, and what measures I would have to take to keep them alive in an aquarium?

Answer
Hello Richard,

I looked into this a little bit more and I was wondering what area you are from, and alittle bit more about what oysters you are trying to study?

Thanks for your time,
Michael M.