Most music stores will have old music for really cheap. I know the store I worked at had a room FULL of old music for about $1 per page if not even cheaper.
Get a bunch of stuff you've never even heard, then try to work through it. It'll be hard and slow at first, but as you practice, you'll pick up on things, like always check key signature first. Once you know the key, you are going to be able to play the piece. Check for key changes. If none, look for any odd notes, and just work from there.