Check out your local thrift stores, and places like hospital thrift stores. Sometimes people are just clearing out their house and toss it all.
Garage and tag sales as well. You might even find some stuff to sell for more on ebay.
I recently went to a local dump, and there is an area there of free stuff - and I found a few books that were in the public domain.
Also once there was a local library that rebuilt their facilities and sold a ton of books for 5 dollars a box. There were old books there and all I needed to do was grab them and look them up (after looking at the copyright).
If you can specify a niche that might help you as well. For example, I discovered some auctions on ebay for discs that had patterns for making some vintage crochet purses. The pattern books could be looked up and found online. I was able to purchase these specific books myself online for less than 20 bucks a piece. I have them in storage somewhere, but I can legally scan them myself and sell my own copies now. Or sell the individual patterns. I was hoping to create an entire recreation kit with them and may be able to get back to that soon.