Sometimes a hard to find book can be just as elusive to capture as the unicorn within its pages, but just as much a treasure if you are successful in your quest. From personal experience, you're most likely to find these books completely by accident in a forgotten little used book store, nearly invisible to the oblivious passerby. Some of these stores cannot be found walking down the street, sometimes you must enter the strange world of electrons and bytes. Hang on tight, for here is a Realm through which even the Archivist walks in wonder.
The "Big Names"
Barnes and Noble
Indigo.ca (good for Canadian customers)
Advanced Book Exchange
Books and Book Collecting
Baen Free Library
Circle of Books
Other Great Resources
SF Signal FlowChart
Note: I don't get any financial support by advertising any of these sites. I also hold no responsability if any of these companies don't come through on their end. It's the Internet, and buyer beware. I myself have dealt with only a couple of them. This page is just to give you that one last chance you need to find the one book you've spent your entire life looking for.
Have a good site to recommend? One of these sites really bad or even a fraud? Let your fellow questors know!