Well for us Australians you must try this very good Australian site Books and Collectibles
Next on the list is Biblio they are one of the largest independent books seller now that amazon owns them.
biblio
The best used book site in the world was AbeBooks but I think they have been managed buy the designers of the dodo.
So I see the end of a truly great site since their take by Amazon.
AbeBooks
Also a very good site in that it lets you look at a lot of other book selling sites BookFinder.
Then you can try Ebay as a last resort.
After this you can spend many hours looking but really from this point it's just luck. Of which I wish you well.
After this you have Amazon
Then Bookfinder4U
Or maybe Biblion
Then you can try the people Bookfound
you can try TomFolio
or you can submit your site to Dmegs Web Directory
A good place to search is ESources.co.uk
Another place is The Australian Index
Finding used books in out of the way places
Okay we don't
always look for books, oh yeah we do. But some times you have to be
expanding web directory. Every listing recieves a FREE screenshot of their website.