I think you'll have to load your own.
Lee used to make a Lee Loader for various shotgun gauges. They are often sold on Ebay at a reasonable price.
I believe Starline makes brass 12 gauge empty cases. These would be good for assembling your black powder loads as they would be totally unmistakable. There would be no chance of putting them in a modern shotgun, where the black powder would foul the action and eventually cause rust.
Dixie Gun Works or Buffalo Arms sell wads appropriate for black powder. Modern, one-piece plastic wads take up too much powder space in the case; you have to use cardboard and fiber wads.
Or so I experienced long ago when I made my own black powder shotgun shells in modern, plastic cases.
Perhaps the newly made brass shotgun shells can accommodate both black powder and plastic, one-piece wads. I'm unsure.
For best results, the
volume of the shot charge equals the volume of black powder used.
That is, if you use a measure that throws 1-1/8 ounces of shot, use the same measure to scoop out the black powder charge.
Not sure how this would translate with buckshot. I guess the closest means would be to weigh how many buckshot you plan to load, then take that weight and equate it to the weight of a shotload.
With that shotload (Okay ...
you in the back ... stop giggling ... I said
shotload 
) weight, you could use a small tube to determine how much volume that shot occupies.
I'd suggest using No. 9 shot, to avoid air spaces. But 7-1/2 would work to get a good idea of the volume occupied.
Frankly, a search of the internet would probably net you all the information you wanted, but if you can't find anything this would get you started.
Seems to me, you could use the same powder/shot charges listed for muzzleloading shotguns, in those brass shells. Plastic shells don't have nearly as much volume as the brass shells.
Remember that black powder creates a LOT of fouling, so you'll want to soak some of your wads in lubricant. Melt a little Crisco, Bore Butter, lard or Gatofeo No. 1 Lubricant (my personal favorite
) and soak some 1/2" thick felt wads in it.
Load the shell in the following order:
A. Primer
B. Black powder charge
C. Hard cardboard over-powder wad, unlubricated
D. Lubricated felt wad
E. Buckshot or bird shot.
F. Overshot wad, unlubricated
The over powder wad can be up to 1/4 inch thick, to protect the lubricant of the wad above from it reaching the powder and affecting it.
However, the wad over the shot must be fairly thin, so it doesn't affect the buckshot or shot in flight.
I'm unsure how the newer brass shotshells are crimped, or keep the top wad in place. In the old days they used a substance called Water Glass. I'm told that running a bead of Elmer's Glue around the edges of the top wad will bond it to the case and keep the wad from moving under recoil.
Perhaps Starline has reloading instructions; if, indeed, they make the brass shotshells. I think they do.
Hope this helps.
