I make most of my rr thimbles, sideplates, trigger plates and triggers, sights, rr ferrule tips, nosecaps. Mostly because I CAN make them, but also the gun doesn't look like it's covered with store bought parts, looks more handmade in other words.
But when you are doing this for a living, I suspect the ready made parts will save you a lot of time. Back in the day, the smith would use many readily available castings, locks and barrels.
But I don't do this for a living, so I can make as many parts as I like and not have to hold myself accountable.