After a good long trudge up and down the rows, I finally found Horace P. Anderson and Louann Warder Anderson's tombstone in Riverton Cemetery, Riverton, Fremont County, Iowa.
It was next to a large plot that was empty except for a stone saying "Calfee", which was the name of their daughter Fannie's husband. It is probably the plot for Henry H. Calfee and his first wife Annie.