What makes you think it is black powder?
It either has to be one of the early versions of the Champion single barrel or early Hercules double barrel. Is is a twist barrel?
Goex is still loading their "Black Dawg" shells with black powder...any dealer should be able to order them for you, as black powder already loaded in ammunition does not constitute a "hazardous" material for shipping purposes. I think these only come in either #6 or #8 shot so you have very limited loadings.
Other than that though, you will need to load your own. Brass shells are the most appropriate for this and really require no special equipment- just powder, primers, shot, wad materials, either a glue gun or some rubber cement, a 1/2" dowel or larger to push in the wads, a black powder measure, and something- like a nail on the end of your dowel to push out the spent primers.
That's about as short as I can make it.
As to question #2...
It is likely that nothing will affect it's value, as Iver Johnson made boxcar loads of these shotguns under their own name and for any hardware store or jobber that wanted to buy a quantity of them enough to private label them with their own trade name. There are many still in circulation and use down here in the south.
