Used books from Amazon. Can usually get them at at least HALF the MSRP, and they're usually in almost-new shape.

Amazon because I can always get the shipping refunded in my area and they give retardedly awesome deals at random.

I usually buy used from amazon or ebay. Or from livejournal.

More common books, I typically get from Amazon. Sometimes for a harder to find book, I go to eBay.

either amazon or abebooks.