If you're looking for getting them for under $20, there are several places online that can be found through Froogle and Shopping.Yahoo.com. Any cheaper than that, though, you're gonna have to go hunting; most of those are common enough that pawn shops might be your best bet. Fry's might be a decent option if you have one near you, as they've been clearancing a lot of their older games.
Hopefully that's somewhat helpful, and good luck in your search.