I love books.
Dover Publications reprints about a billion older text books plus a ton of copyright-free clip art books. They're also known for cheap classics as well as a number of abridged children's books. If you like getting free stuff, you can actually sign up to receive a link to weekly free clip art. I don't know what you'll do with it, but it could be used to practice art lessons (copy this, sketch that), or design your own cards or stationary.
If you love old library books (as I've recently decided I do in my attempt to corner the market on "How Did We Find Out" books and "How and Why Wonder Books"), then you can't beat Amazon marketplace ('see other ways to buy this book' button) and Campusi.com. Enter a book search term and Campus i will check out all of the book sellers to find the book (used or new) and then display their price, even with shipping. Great resource I discovered from a local homeschool friend.