Do you have pictures of the snails? Many asian markets will carry live whelks, a popular asian treat. Some Chinese stores will carry various types of fresh water snails, but due to import rules, most will be cooked or prepared in some way. But with some photos, identification is pretty easy.
How about fresh water clams? There are a variety of freshwater types, from the standard golden clams to things like domestic pea clams. One should stay away from live mussels, especially fresh water ones, as mussels have a parasitic stage in the life cycle. I have read many folks that think clams also have a parasitic stage, but freshwater clams simply dump young out at a prolific rate fully formed.
https://dnr.wi.gov/water/basin/mississi ... Mussel.pdf
This is a good detail on the difference. If you buy a bunch of freshwater clams though, and inoculate a tank up, remember clams still need food, they are not plants... they do not survive by eating the nitrates they filter from water. They are filter FEEDERS, not filters. The water they filter needs to have real food in it
They can live a long time though while starving to death, so if you raise them, read up on the species and feed them as well be prepared to do a health check on your stock.
There are also dwarf crayfish. I am raising dwarf cajun crayfish now, so I have plenty when the time is right.
Right now Snoop, my little baby fahaka, is happy chasing scuds and chowing down on BB sized snails. I have a set of feeder tanks already going though for future needs. Every tank has lots of snails, but a couple scud cultures, a tank full of golden clams, another for crayfish. And lots of guppies going all over. I am hoping I can generate the right sized food, at the right times, to keep Snoop happy and healthy.
Some pictures though of those market snails will help identify them up!