Actually, you can usually get the snails you'll want for free--they are common hitch hikers on plants. What type of puffer do you want? Most--F8's, SA's, leopards, etc will require a daily feeding of crunchy food, so raising enough snails can be pretty intensive and require a bit bigger tank than you might expect. On the plus side, snails are hardy, so the tank doesn't have to be immaculate, but you'll need lots.
If you're looking at the dwarves, they are considerably less maintenance. They need to have some snails in their diet, but do not experience the same degree of tooth growth as the larger species. They are also true FW, while many of the puffers sold as FW will require brackish to marine conditions as adults.