I buy all of my books off used book stores online. Google for "used books". You will get a range of websites. Most of them will sell the book for 1.00$, and only make some money off the shipping (around 3$). AbeBooks is a pretty cool site... just google and compare prices.
Yeah, and if you use AbeBooks and are buying several books at once... Make sure to buy them from the same seller (AbeBooks compares prices for books from different used book sellers)... that way you will probably get a shipping discount. If AbeBooks don't show one, then go directly to the seller's website, and buy directly through them and not through AbeBooks.
Ebay is good, I've got lots of bargains off there including a first edition of Terry brooks's First King of Shannara for £13 ($20) And AbeBooks is awesome too, especially for rare and out of print ones.
I'm from the Pacific Northwest, spent a lot of time in Portland, and I LOVE LOVE LOVE the original Powell's store. It is, without doubt, the best bookstore I've ever been in. That said, I have no experience with buying form them online. Might not be the best option for CHEAP, but in terms of selection/rare books/awsome business policy the are worth it to support. I've known a lot of people to buy cheap used paperbacks (in decent condition) on half.com. there are a lot of titles available for $1-2, but then you'll have another $3 or so on shipping. It all evens out, really, if you have a good used bookstore local to you.
I love Powell's as well. allbooks4less has been good to me in the past. But, I still like to buy books at used book stores best.
www.bookfinder.com - searches multiple independant and chain booksellers www.half.com - now part of eBay, but this link takes you to the right section. Selection varies a lot, but it's hard to find cheaper books.