Hi Michele, If they were buried on a ranch on private property, there graves might never have been recorded. I'd contact Random Acts Genealogical Kindness, raogk.org/californ4.htm, read guidelines, choose California, choose Sonoma Co and choose David.
If he can't help you, then contact Krystine.
Also contact the Sonoma County Genealogical Society, scgs.org/scgs.html . Successful Searching!