Hi Maria,
There are two obituary volunteers in Missoula:

Obituary Lookup Volunteers, freepages.genealogy.rootsweb.ancestry.com/~obitl/volmt.html
Scroll down to Missoula Co.

Montana Free Genealogy Look Ups, www.geneasearch.com/lookups.mt.htm

If you decide to order a copy of her death certificate, contact the Missoula County Clerk & Recorder Dept., www.co.missoula.mt.us/clerkrec/ . Successful Searching!