You can find them online from some sites. They're fairly rare in stores, and expensive for a snail. They're slow breeders, and don't eat quickly. Just about any loach will eat snails as well.
Snails only breed to match the food available. They're not great at competing with fish, so if they're getting enough food to reproduce out of control, you're overfeeding. I've had several species of snail breeding in my 55 gallon planted tank for some time, and they're rarely seen anymore. I think at least one of the species has died off without me doing anything. When I first got them, I had good success getting them under control by putting lettuce in the tank overnight and pulling it out, snails and all. If you don't have too many, you can take a fork and crunch their shells, your fish will do the rest.
