Mennonite deaths

G-grandfather Martin Shellenberger, died between 1850-1855 in Trumbull Co, Ohio.  I have nothing else but this fact.  I would like to find his grave site, but his death was either not recorded in Trumbull Co or he was buried in a Mennonite cemetery, which I can not find.  His wife, Nancy Strickler Shellenberger, died in Ashtabula Co, Ohio in 1872.  I know this because of a newspaper article.  I can find no grave for Nancy either.  I have talked to the Ashtabula county offices, but they can offer no further information on Nancys burial.  They THINK there are Mennonite cemeteries around the area, but they are not sure.  Any ideas?