Most military pieces use lead.
I flattened a .75 ball to hold the flint in my Bess and it's just the right size. To the best of my recollection the same piece of lead has been in there since the late 70's and still working. You can buy pieces of sheet lead from the suttlers that are cut to wrap musket flints with but they aren't necessary. Leather works but it doesn't always hold up to the strong blow of a musket lock and will allow movement.