Gatsby, it's the only way to go!

I typically make my own punches, although I also have some industrial 'arch' type punches as often seen in gun-case tool kits. I also have some with bore markings (24, 16, 10, etc) which likely came from the gun trade.

For drill-press punches, you can use store-bought hole saws and grind the teeth back and sharpen the end of the remaining tube. The wall thickness is thin, and if you don't grind all the way through the high speed steel end of the (bi-metal) hole saw you will have a very tough cutting edge.
I turn them in the lathe with carbide or ceramic cutters.

Such a hole-punch spinning in a hole saw arbor can make quick, accurate, and repeatable work of the wad or gas-seal cutting process.

Again, it's cheap and easy and it gets you set up for the long haul.

By the way, McMaster-Carr ( mcmaster.com ) stocks them in 15mm.

Cheers
Tinker