Enjoy the percussion shotgun. In the old, usable originals I have used Triple 777 ff powder. The functional consistence is enhanced with the black powder substitute. In a 12 ga percussion my loads have been 2 3/4 drams of that powder, an ounce and 1/8th of lead shot. A tight fitting over powder wad, half and cushion wad, shot and an over shot wad are used. For 10 yard trap 2 1/2 drams of powder and an ounce of shot will work well. For sustained use as in trap or skeet the 777 in old guns does the job well and it is easy to clean afterwards. Real black powder in the old guns works well in occasional usage, but can foul up barrels in sustained shooting. At one of the Friendship trap matches a few years ago I used an original 12 ga. Birmingham double with black powder. Towards the end of the match it was fouled and the match could not be finished.
Don't buy wads. Go to Harbor Freight and buy a set of their hollow punches. Use the cardboard in cereal boxes and punch out cards. A stack of 4 or five serve as an over powder wad and one card will serve for an over shot card. Ack, the set only goes to 1/2". There should be sets that go to 3/4" diameter.
Don’t buy wads, just ball up natural fibers brown paper, tow, jute , sisal, or hemp rope, cedar bark, old woolen scraps, if it’s easily flammable a tough of grease will stop that