Thrift stores are a given. Also ghetto department stores that sell post-retail inventories, if there are any nearby, like Ross, Kohl's, Burlington Coats, or Big Lots might be good place to look. I've found some fine deals on shoes at Ross.
Try the thrift store, they usually have used shoes in good condition for $5-20. Try Claire's, Winners, and Stitches too. I believe they all carry a cheap knock-off brand of converse called Streetwear Society that run about $20.