I've been looking myself. I've seen plastic ones and wood ones at Wal-Mart, Hobby Lobby, Michaels Crafts, etc....but I've not seen metal ones yet. I heard you need a 9" one, but the plastics come in 8 + 10", and the wood ones come in 9", but are very flimsy for what it's needed for with a Fett helmet.
I got mine at ben franklin crafts, it looks like a 1/8th inch metal in the form of a 9"ring. then one I got was brass but I'm sure can come in nickle if you prefer.
On another note, they really are kinda ghettoed in the stores, they dont have a package just a price tag sticker.
THe rubies helmets have a problem holding their shape, you cut the rind to fis in the back of the helmet to keep it round. I think that's really the only helpful thing bloodsport productions has shared with the newbies.