Free: Contests & Raffles.
Many years ago I thought about adding a SxS caplock 12 gauge to my muzzleloader stable. Mainly for pheasant and grouse, but also for those bluebird duck days that you don't expect much shooting. If I recall, Pedersoli had a few field doubles that fit the bill (steel safe) for a very fair price. Not sure if that's the case anymore, but might be a good place to start.