I asked much the same question not long ago, and a wise invertibrate expert told me to go to the nearest Wal-mart or whatever and pick up a cuttlefish bone - the kind you give parakeets and etc. to wear down their beak. They're super-cheap - maybe a buck or two.
Buy one, remove the metal clip, break it into chunks (they break really easily) and drop a piece in the tank. It won't do anything to your pH - it's pretty much non-reactive - but the snails and shrimp *will* visit it.
I have a small tank just for ramshorns, and I've had really good luck with this. One day, I hope to have time to try to make snail jello. But for now, slices of cucumber and pieces of cuttlefish bone seem to fill the bill just fine. My snails look great.