Our local friends group makes an effort to keep books that would be useful to homeschoolers together when they have their book sales. You can get great books there for $.50 - $1.00.
Here is my discount "library" it's called Library and Ed. They offer homeschoolers and non-profits (church teachers/leaders etc) wholesale prices. Our library is very weak. My mom's library is now charging an $80/yr membership fee
I've gotten some cheap and great resources from this site. (clearance items rock!)