I just got an email from a teacher about http://teacherexpress.scholastic.com/ They have $1 books! I got a couple (or 20) from the Pre-K and Kindergarten section. They are downloadable, so you get them instantly. I just saved all of mine to my computer and got a chance to glance through some and they seem to be great resources, especially since they are $1.
The catch...the sale ends tomorrow, so get on it!