Make sure you keep an eye on what kind of snail you aquire. Common ramshorns seem to be the best as they have a small adult size, grow like crazy, are fine in room temperature water and are prolific as all heck. I put 6 in a 5 gallon tank a couple of months ago and now I have to cull the ones that get too big for my puffer as they will cover every avaliable surface with egg sacks seemingly overnight (I've got a feeling that a couple thousand snails will overload the tank pretty darn quick). I have a bag of krill in the freezer as puffer food, but so far I havent even had to open it. My puffer has an all snail diet at the moment and he loves it.
The first mistake that I made was buying some "columbian ramshorns" when I first tried to set this up. they have the same general ramshorn shell shape but the columbians have an operculum which covers the opening of the shell (they're a type of mystery snail). Its not that a puffer wont eat these, its just that they're tropical so require a heater, dont lay as many eggs and seem to grow very slowly. Apple snails are also not a great choice for the same reasons, they also get the size of a baseball, so even a couple in a 5 gallon tank for breeding purposes is not the best.
Go to your local big box pet store (Petco in the US or Petcetera in canada) or any LFS with a good number of live plants and they should let you take as many snails as you want for free.
