AFAIK, all commercial copper gas checks in this country are now made by Hornady, even if sold under Lyman or another brand name. If they don't have one for 41 Mag, you will have to make your own. In that case, go to the castboolits.com forum and seach on gas checks or making gas checks. Their is at least one thread I read there that had the name of the person making and selling the tool for making gas checks from aluminum cans. You can also buy copper foil, of course.
An alternative is the P-wad. It does take up a 1/16" of powder space, but stops leading completely, as near as I can tell. That assumes you don't have an extra rough bore.
An alternative is the P-wad. It does take up a 1/16" of powder space, but stops leading completely, as near as I can tell. That assumes you don't have an extra rough bore.