Also garage sales, thrift stores, Half Price Books (especially their clearance book sales – ours was at our State Fair in Sept, I think, this year). Abebooks.com is another I go to. Rarely do I find something cheaper on Barnes&Noble.com, but it is worth checking – I occasionally do find something just a little bit cheaper. Sometimes homeschoolclassifieds.com has a living book or 2, as well.
I also gave a booklist to my mom, who is a bookaholic like me. 