Scholastic is the leader in children's books. You can check out their website. Within the next month the elemtary schools should be having their spring book fairs. You may want to stop in and take a look.
Check out www.arbookfind.com. (not for purchasing, just for info - ratings by kids, grade levels, and difficulty) This is part of the Renaissance Learning program that is being used in many schools. Send me a PM if you want more info.
If your interested in used books, you may want to check out www.paperbookswap.com. Which is books for everyone-kids,adults, students. It's basically a program where you swap books, but you can also purchase them.
Happy reading.