Most will give them away at the LFS. Here, you may just have to pay the cost of shipping. There are plenty of folks out here who are more than willing to send you some.
local stores, or pay shipping on here . If you need a constant supply look into setting up a snail breeder so you don't have to always be getting them from somewhere else.
what thebrandon said. Take an old bucket or big glass jar or anything that will hold water and throw some plant clippings in it and seed it with some pond pests and ramshorns. Get a can of the cheapest fish food possible, junk you would never dream of feeding to a fish you care about, and feed heavily for best results. within a couple of months you will never have a snail shortage again.
better yet send me a couple of those assassins. I got one awhile back but I think it OD'ed on MTS cuz it died.
Just ask for some when you go you LFS next time, or buy some plants. Most plants always have some snails/snail eggs on their leaves that you don't notice until you bring them home.
I have all the plants I need for the next century! Right now I'd rather pay for the snails than more plants. I do have some growing in a few breeding tanks but they are months out from being ready to feed.