Just to add to djlen's post:
Use 1 tablespoon of alum per gallon of water. Soak the plants a minimum of 20 minutes, up to a maximum of 2 hours. A two-day soak will kill snails and their eggs. Avoid using this method on extremely fragile plants.
I find alum in any grocery store with the spices or in the canning section. It's most often used in making pickles (keeps them crisp). It's a white powder.