Genealogical and history books are normally printed in small runs. They go out of print and sometimes are not reprinted for many years, if ever. The old saying "The time to purchase a book is when you see it" is quite relevant among these publishers and re-sellers. Check periodically.

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Barnettes Books - Oklahoma

Green's Used Genealogy Books - Oklahoma

Frontier Press Bookstore - Oklahoma

Heritage Books

Willow Bend Books - Oklahoma

List of Oklahoma Genealogical and Historical Societies
Check for publications list.