Stores that have them usually sell them for a couple dollars, but only one or two snails is all you need. Melanoides tuberculata
is the scientific name, though other actual species may be within the common Malaysian Trumpet or Livebearing Snail group. These are Prosobranch snails, meaning they have gills, and they also have an operculum which is best described as a trapdoor that shuts and protects them from fish predation; loaches find these snails very difficult to "get at."
Some sources say this snail is an hermaphrodite, meaning each snail possesses both male and female organs and can thus create and fertilize its own eggs, while others say there are distinct male and female snails. The "parent" retains the eggs inside the shell until it hatches, and then the young snail is released. This snail lives 3-5 years, compared to a normal one year lifespan of other freshwater snails.