Visits to different stores and street vendors in Chinatown can yield all sorts of turtles, but I think by volume and number, I've seen, heard, and read about red-ears most often. My wife actually bought a pair as pets, not realizing they were being sold as food, or that they were illegal to buy as pets until 4 inches, since supposedly the government is worried about kids putting them in their mouths and sucking the salmonella out until they're larger. Two Chinese teacher-friends set her straight, and I researched the species (they were sold to her as, wait for it, "luck turtles"). They were awesome pets, grew quickly, and were passed on to a friend of the family in need of companionship and with much more room than us.
I typically audit a few storefronts per Chinatown trip, and while I've seen everything else described above, there is usually also a huge bin of tiny sliders ready for the picking, much less expensive and easier to get home and store. I'm not thrilled about this. I'd much rather have a hunk o' snapper.