Check the planted tank at your lfs. There are usually a ton of "pest snails" in there. Ask if you can have some.
Many stores will just let you take them, some will charge you, but not much.
To keep the pest snails breeding, you're going to need a separate tank, as most pest snails can't handle brackish water.
If you don't have a freshwater tank that you can use, a snail only tank can be as small as 2 gallons, with minimal filtration, and routine feeding. Keep the snail tank fairly bare (makes it easier to catch the li'l buggers).
Make sure you put some fine mesh over the filter intake so you don't suck up the babies.
You should have snails out the wazoo and one happy puffer in no time.