One of my favorite ways to get cheap books is to use paperbackswap.com. You post ten paperback books and are given free credits to begin trading with others who want to trade books. I’ve been using for a year now and haven’t been burned yet!
As a teacher, I use it for:
AR books for my classroom library
Books to give away as prizes for contests and good behavior
Classroom Management or Curriculum books that extend what I teach.
I have found tons of teacher journals and scholastic teacher resources this way. I would never have afforded the wonderful teaching library I have now if it weren’t for Paperbackswap.com.