Hi Jan, I did a web search on Highland Cemetery Colville Washington and it took me to that Cemetery. Mary is buried there. You can find it at Interment.net, www.interment.net/us/wa/stevens.htm and click on Highland Cemetery.
For a copy of her obit (if one was published) go the the Secretary of State, http://www.secstate.wa.gov/library/ask.aspxhttp://www.secstate.wa.gov/library/ask.aspx. They have the newspaper microfilm for the State. Be sure to tell them she died in Walla Walla but was buried in Colville. They will check the newspaper microfilm for one hour at no charge & reply with what they find. Successful Hunting!