Cheap Books

I know some of you are readers. I am looking for places to buy cheap books in case I finally get a teaching job this year. I need kids books, though. :) I would love to be able to provide my students with a large selection of books to read during independent reading time in the classroom. I already have 2 or 3 Rubbermaid boxes full of books but they range in reading levels and some are very nice and I am not willing to put them out on the shelves. I'll only use those as read-alouds. Anyhow, If anyone has ideas of where to buy super cheap books that'd be great! Thanks!!