Often times fish stores are glad to get rid of some of the pest ones that they have. It's worth it to check it out and then set up a simple small like 2 gallon to "raise snails". Then you'll have your own supply. I've heard many conflicting opinions on malaysian trumpet snails. Some say that they can break their beak on them, but a knowledgable person said they only bite the foot off so it's not that big of deal.
I've heard quite a few times that pond snails could be the best. For a treat every now and then chuck in a little shrimp if you want (what kinda puffer do you have). They will eventually eat it.