silver
You might be able to punch silver, but much harder with nickle silver.
If you can't mill them , here's away to do it with a drill press. Get a sheet of the material in the thickness you need. super glue it to a wooden block. Drill the outside with a hole saw.
Drill the inside with a silver deming drill. Then cut away as much of the wood as possible.
Dunk it in acetone and the glue will release. Pain in the butt, but can be done with
hobby tools. I had a bunch( min order) of .060 thick done for George cues, I mill the .050
thick (Gus size) . I don't know anywhere to buy just a few.
At .050 thick, nickle silver is a whole lot less exspenive than .999 silver.