FindAGrave, and gave me permission to post them here.
According to the gravestone, Elver was born 23 April 1891 and died 5 December 1980. Isaac was born 24 October 1886 and died 10 October 1965, a little more than 44 years ago. These death dates are further bolstered by the Social Security Death Index.
Well, I think that's a good start. All of the above information was found free of charge on the Internet. I think I found an obituary for Elver on NewspaperArchive. I will have to subscribe soon and review it. I've been wanting to give them a try, but was waiting for a search to provide likely fruitful results.
By the way, I am not directly related to Isaac and Elver. They are, however, part of a collateral line intertwined in my family history.
One final thing, I added to Isaac Zumwalt's page at Footnote.