Beleive me when I tell you its literally impossible to find parts for these. You really only have a couple of choises. Either purchase one for parts or make your own.
I'm experimenting with making the other type "cap". First one works/looks good, but doesn't have the exactly the right color.
If you have another one, or know someone who has one like this, I can try to cast one for you.
Can't remember...does that take the disk pendulum? If so, just take a hermle #41 mainspring barrel do any necessary turning/drilling and fill with lead. I can give you more info. if desired.
The Cylinder pendulum is just a 1" dia. X 1 3/4" solid brass...drill/tap through the cylinder and make two turnings on top and 3 on bottom and your done. I think I've got one, but can't seem to lay my hands on it right now(so much for being organized).