This is the best well for me but I'm biased 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.
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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
resourceful so looking in other ways is necessary.
Google is great but sometimes you have to look further afield.
So here are some of the ones we like to try.