check this
http://www.youtube.com/watch?v=6sKBsp77PY0
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check this
http://www.youtube.com/watch?v=6sKBsp77PY0
amazon and ebay.
My library sells all their old books they have extras of for 25 cents per paperback and 75 cents per hardcover. You could always check and see if your library does anything like that. If you like a book enough that you would read it multiple times then buy it -it's typically worth it in that case, but if not you should just rent them.
You could also try Craigslist, sometimes people have an ass-load of books that they sell for really cheap just so they can get rid of them. Thrift stores also, even rummage sales... stuff like that.
I usually check Half.com first, then Amazon resellers 2nd. If you're planning on buying multiple books at once, then just stick with Amazon, since you'll save with free shipping.
ebay
You could try your local freecycle group. Just google freecycle followed by your area/city/town etc.
ebay