Look for snail or their eggs on plants for sale at your
LFS. It's common to get snails as hitchikers when you buy aquatic plants. Once you have them, it's usually hard to get rid of them, so you won't need that many to start with. You could easily get one of those plastic 1 or 2 gallon tanks, and breed them for your puffers. You would just need to provide filtration and some food like veggie wafers. They will also eat dead leaves off any live plants you had in the tank.
P.S. You could also ask if you could have the small snails for free. Most hobbyists and lfs consider them a nuisance, and they might be happy to give them to you.
