If they died before 1817, that would have been in St Tammany Parish records
good luck in your search
http://www.usgwarchives.net/la/sttammany/cemeteries.htmSt Tammany parish original parish before it became Washington parish
http://www.usgwarchives.net/la/washington/cemetery.htmWashington parish was incorp. after 1817