craftgrrl | getting into book alteration...

where can I find an online store or something that sells old, worn out, unwanted, used, hardcover books? I tried searching half.com and mysimon.com but I don't recieve very good results on searching "random used books" or something similar...they only find specific ones. I don't care what kind of book it is as long as it is hardcover and thisclose to being chucked in the trash...I would feel bad using a new and often-read book.

thank you!!