What are the chances of getting a pair of breech plugs fitted to an antique muzzleloading 12ga double barrel? Where should I start?

Depends on the age of the piece . If its say lat 1860 then you might get lucky and have cut threads . Earlier you go the more common were threads made by forming dies or just hand filed . In which case your would have to either re thread or hand file a new set of plugs .