(1) I assume that your school puts out a prent newsletter? Run an ad asking families to send in any books that they no longer need for their older kids.

(2) Stage a contest asking kids to bring in their unused books. Arrange a prize for the grade level or home room or whatever that brings in the most.

(3) Is there a Barnes & Noble near by? They'll do a promotion day or night for you or your school during which your families can come by and help. You can create a "wish list" from which, ideally, the families would buy books for your room.

Here in Massachusetts Scholastic has a warehouse. Once a year they have a big sale to get rid of overstock. There might be something similar going on down your way. :)

Ever heard of Freecycle on the internet? People in regional areas post stuff to give away for free, or stuff that they're looking for. Here's the SF one:

groups.yahoo.com/group/SFFN/

And here's the tribe for it:

freecycle-sf.tribe.net/

Let them know you're looking for 3rd-grader books.

I honestly found Scholastic Book Clubs is the cheapest
way to get new books for my classroom. I write a letter to
parents with a recommendation list blank on the bottom
and then write a list of books I recommend for their child
from that months Scholastic catalog. I offer incentives to
kids for bringing in the biggest order, prizes and such.
If I don't do this too often it works pretty well. If you earn
1000 bonus points you can 100 free books from Scholastic
although they pick them for you.

Another thing I do is instead of having a gift exchange in my
classroom I ask each child to bring in a new, wrapped book
to give TO the classroom. That's a quick way to get quite a
few new books and you can even give a Wish List for your
classroom library to give parents. It works very well and
directly benefits their children too.

Vixxen