Roger, the cheapest way - go out and catch them yourself. With the snails it should be easy (especially in fall), for the frogs you have to be a bit faster...and some frogs are extremely poisonous. To be honest, I couldn't kill anything bigger than a midge, and definitely no creature in whose eyes I could look (with one exception: to save them from suffering). Nevertheless I do eat fish and meat, and it troubles me sometimes that it's just possible because others do the killing.
However, I bet these are two of those dishes originally created by poor people who couldn't afford meat...