Using lead used to be a fairly common practice. Leather is more common these days.
About 10 years ago I took my TC Renegade to a friend's, but I couldn't find the stinking leather for the flint, and he couldn't find anything suitable around the house.
We were just about ready to hang it up in disgust when I remembered that musket flints were often head in with pieces of sheet lead.
So I took a .50 cal. ball, beat it flat on an anvil, and used it instead.
I've never gone back to leather. The lead held the flint more securely than any piece of leather I've ever used.
Additionally, you can simply wash the lead off when you clean the gun. With a piece of leather, you have to remove it from the gun or the powder residue that gets impregnated into the leather can cause the cock jaws to rust like crazy.