Dear Mr. Steele,
Thank you for posting your request on History Hub!
The Federal Government does not ordinarily create or maintain birth, death, marriage or divorce records. Such records are made and kept by State and local governments rather than the National Archives. We suggest that you search the Montana Deaths, 1860-2007 FamilySearch index and review the FamilySearch Research wiki for Montana Death Records.
We hope this is helpful. Best of luck with your family research!