Metal disks availability?

Almost any metal manufacturing facility will have at least one punch press machine that will turn out "slugs" as holes are punched in sheet metal. These slugs are captured and dumped into hoppers to be sent to metal recycling facilities. I am guessing that you can easily find what you are looking for by visiting these businesses and doing a little exploring. Many of these plants are closed on Sundays and their facilities locked behind chain link fences. Some are remarkably accessible. In either case, these slugs spill and drop from the dumpster type containers when they are hauled away by the recyclers. Check along the driveway to the "yard" area of the plant and you will find what you need. Do not expect to find brass since steel is much more common.

Good luck with your search.

You could try a machine shop. They might have something like this they use for shims on different projects.