Montana Historical Society

The Montana Historical Society (MHS) is a historical society located in the U.S. State of Montana that acts to preserve historical resources important to the understanding of Montana history. The society provides services through five operation projects: Education and Outreach, Historic Preservation, a Museum, Research Center, and Publications. Founded in 1865, it is the oldest such institution west of the Mississippi (excluding Louisiana).

More information on the Wikipedia page.

The website is.