In the U.S.; my first thought is DuPont De Nemoirs. They supplied most, if not all, of the U.S.-made powder for the American Revolution as I understand it. I think that is who GOEX bought their Louisiana powder mill from.
The percussion cap, Minnie ball ... both French, as was DuPont.
I would look for somebody in Louisiana first, either a manufacturer or dealer. The Mississippi River right there in New Orleans was a major trade route then and now. Easy to get stuff from the industrial northeast and mid-Atlantic down to the south.
