There is a website called www.aribis.com, I may be wrong on the spelling.
A classmate in my chilren's lit class said someone in her county sells books by the boxes on ebay. She said they are good books and some that had ordered some says they come out to around $1.00 each.
I've bought a few books at Goodwill and consingment shops. But my biggest weakness is The Book Gallery. I am not sure if it a national chain or not. They sell liquidated books CHEAP. I've even gotten my boys some popular books on tapes for $8.
Have you ask B & N or someone like them to donate few?
I am a huge fan of garage sales and have picked up hundreds of brand new books for pennies on the dollar. I have singlehandedly filled 3 bookshelves in my sons classroom library.
In our area, they run in spring and fall mostly. Start collecting for next year.
For our Family Fun day we have a Book Walk. We ask each teacher to donate a new book to be used as a prize. The support that we receive from them is wonderful. Most of the faculty donate more then one book. We are a school of approx. 950 students grades k-5th and always have more then enough books.
Ebay can also be a great source for finding lots of new books at great prices. Last year for a class party I was able to get 30 brand new Halloween books for $10 with shipping and handling. That made them cost only 33 cents each.