I'd contact the Museum of Upper Missouri, Fort Benton. They might have early documents or know about early newspapers.
Also the head of most U. S. forts kept a diary and would record the births, marriages and deaths. I'd ask the Museum about it too.
If they don't have it, I'd contact the following:
Montana Historical Society Research Center, Http://mhs.mt.gov/research/
Montana Memory, www.montanamemory.org
who has digitized newspapers, prison records, photographs, etc.
Montana State Genealogical Society, www.montanamsgs.org