I've actually had a lot of luck at secondhand stores that have bought overstock or stock from businesses that have gone under. I found each figure from the Blob series Marvel Legends for $5 at one, and a few other assorted ones as well. Wal-Mart's will selectively have good stuff, but nowadays all I really see at mine is Transformers and Hot Wheels. ):
Amazon is another decent place, if you want to order online. Usually their stuff is a little pricey, but every once and a while some store will post a good deal with the free shipping thing. I recently got a Madame Hydra for $18 *Fist pump*
Ebay is about like Amazon, you've just got to check in often if you want to find the really good deals on the things you want.