Check the Scholastic books web site to see if there is a warehouse near you. A few times a year they have warehouse sales that are open to the public. The savings are incredible.
To boot, if you register on the web site, you can print out a coupon for $10 off your $50 purchase.
And, at some sales you can volunteer in exchange for sale credit.
I volunteered at one have and ended up buying 15 books for 47 cents!