Our Research By Year pages detail what records are available during specific time periods. For 1848 see the 1840-1849 page. As a death during this time period pre-dates Vital Registration you will have to seek out church and cemetery records.
Also see our links page for a link to the genealogy group that covers the area where the death took place. Many genealogy groups have published transcripts of local cemeteries plus have a wealth of local knowledge.
As well be sure to check out the Ontario Cemetery Finding Aid and CanadaGenWeb's Cemetery Project.