According to Riverside Cemetery records (page F3),
Alexander FORBES died 19 Oct 1935.

Death certificates now cost $15 each.

South Dakota Department of Health
Genealogy Resources--http://doh.sd.gov/VitalRecords/Genealogy/default.aspx
Beadle County Resources--
http://doh.sd.gov/VitalRecords/Genealogy/Beadle.aspx

Beadle County Official Website--
http://www.beadlecounty.org/

Sheryl Stearns Hickle
Beadle Co. SD Genealgy & History