I also find some great deals on books at
www.bookcloseouts.com
It includes a wide range of books from a variety of publishers (including Scholastic).
Of course browsing for books is a unique shopping experience. My first choice is always to be able to hold and view books I'm considering as gifts. So online purchasing may not be the answer for you.
But if I already know the title, or perhaps the teacher has a wish list of books or authors, this online resources can be pretty handy.
They frequently run coupon codes. The current ones are:
Coupon: 5off25
Password: BookCloseouts
Value: Take $5 off your order of $25 or more
Coupon: 10off50
Password: BookCloseouts
Value: Take $10 off your $50 order or more
The current "Children's Booksale" offers
10 books for only $15.00
20 books for only $25.00
50 books for only $50.00
100 books for only $75.00