I have a couple of sites that I use quite a bit to complete sets in our library. The first is www.cloydsbooks.com . This is an individual who buys retired pastors libraries and offers them to the public. Periodically, he offers books for as little as a dollar a piece and he picks up the shipping. I can't tell you how much this has meant to our library, it has really helped stretch our budget. Another site I use quite a bit is www.bookmooch.com this site is a great way to trade books that you multiple copies of in your library. The short story about this site is that you can search through thousands upon thousands of books and trade your books for something that might interest your patron. After you list the books that you will part with you will be able to get books from someone on the site. no cost to you, just send them an email and they will send it to you. In return if someone wants your book you will send it to them, you pay for postage on the books you send out.