Unknown Brokerage account for old stock positions

My fiancée has stock positions from the 90s – old enough that she has the physical certificates – that were given to her by deceased grandparents. Unfortunately she doesn’t know which broker/account these would sit with. They are set up for dividend reinvestment plans, and she receives mail on this – but we can’t find any information on where/who the positions sit with. Any ideas on how one would go about selling these positions if you don’t know who the account is with?