I would try to find a cartridge of the appropriate size and cut a piece off of it. It is difficult to tell from the photo what the diameter of that is. I save cartridges that I pick up, or ruin some way, and then if I need a ferrule or band, I just cut a slice off. You can get nickeled cartridges in .32, .38 and on up. I have some file handles with what was originally bases from 20 ga shot shells.