Totally Scholastic book clubs. They always have 1 book in every catalog, every month, for $1. So if you order from all 7 of their catalogs, just the $1 book, but do it every month, you can really amass a library quickly.
Plus, they will let anyone sign up for the catalogs. If you home school, that's fine. If you want to sign up your child's daycare, you can. If your school doesn't do book clubs, see if they will. Ask around your neighborhood, social circles, and see if anyone gets catalogs. Then, you can order online through their school and order from any of the available catalogs. Shipping is free. And teachers earn bonus points from the sales.
I love, love, love Scholastic. The have great variety and pretty darn good quality for really inexpensive books. There have been a few books that had middle pages come out, but a quick fix with tape makes it good as new!