Hi Mary, The Malheur County Clerk should have copies of old death certificates. Contact them at www.malheurco.org/countyclerk. If they say they don't, ask them which department does. Their web site doesn't mention copies of death certificates but the Oregon Archives listing for Malheur Co. death certs starts with 1925. Joseph's burial location should be on his death cert.
I see from the 1900 Census record that Joseph was married so there might be an obituary depending on how long they had lived in the area.
There is a Malheur Co. obituary volunteer at freepages.genealogy.rootsweb.ancestry.com/~obitl/volor.html Scroll down to Malheur Co. Maybe she can look up the cemeteries in Benton?
I looked through the online transcribed cemeteries but no Joseph Decker. There aren't many transcribed and on-line.
You can contact the Malheur County Historical Society, 1186 SW Sixth Ave, Ontario, 97301 or 503-889-5073 and ask if they have any additional transcribed cemeteries in Ontario or Benton. Maybe they'll go out to a cemetery and check for you for a donation. Successful Searching!