
ReviewCopy.com bills itself as: "The premier full service academic representative dedicated to serving your teaching, curriculum, research and publishing needs." It has a search engine as well as an "out of print" search.
If the book is fairly recent (within the last few years) search B & N: Out of Print.
If the out of print book you're looking for is not recent (i.e., more than 5-10 years old), try Advanced Book Exchange (ABE). The nice thing about ABE is that you can establish a personal "want list."
Try the "Advanced Search" option at Alibris.