i bough plants from all my lfs and the ones that said are snail free, werent so i had five snails with each plant i got and im prety sure ive seen eggs on the glass just get 3 or 4 and they should breed quickly
when I need cheap snails I buy plants from the LFS from whatever tank I can see with the most snails. Doesn't matter the plant. They get $3.99 for most snails, and each $2 plant has a half dozen on it.