Where can you find book reading levels?

Look on the cover/inside the few front pages of the book. If you can't find it stated on/in the book (not all books will say), try a search for the book online (check the publisher's website?) and hopefully you'll find what you're looking for.

If you're looking for books at a certain level, a simple google search should find what you're looking for.

Addition: Lots of schools use the Accelerated Reader program which gives reading levels to most books so that kids can test appropriately.