You should contact Ft Union directly rather than Maps. If they don't have the book they may be able to help you anyway. The staff is extremely nice!
They have a big selection of books, several of them are out of print and hard to find elsewhere. Call between 9-5 central and they would be more than happy to help
Items for purchase are extremely limited.
You'll likely need to call the Trading Post staff directly and ask rather than asking on Google Maps. I think their merch is pretty limited
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