Would flat brass disks be an acceptable compromise? If so this company sells 3/4" (19mm) 20 SWG blanks in packs 10:
https://www.copperenamelshapes.com/b...eatured&page=2
It's the first item on this page and the postage charge is reasonable. You could always share the surplus through the forum. If you wanted to form them into a concave shape you'd probably need slightly larger discs.
Just trying to think creatively.
Alan