Originally Posted By Ohio_Bill: Have you tried abebooks ?
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This.
Posterity! You will never know, how much it cost the present Generation, to preserve your Freedom! I hope you will make a good Use of it. If you do not, I shall repent in Heaven, that I ever took half the Pains to preserve it.---John Adams
ThriftBooks. Have an app, every fourth or fifth book is free (up to $5 but they have tons at that price). Cheap and have tons of obscure and out of print items.
Thriftbooks and AbeBooks are the two I use the most but, I have learned that you can't trust the photos they use for the items. Stock images are used quite often so, if it's a very particular version I'm looking for, I use ebay.