Most of the levels using this design that I've seen used metal plugs, brass being common if the other metal on the level was brass. As to where to get them - Sargent's out of business. If you want to make them, your hardware store or a hobby shop will have brass sheet (get something fairly thick, like 1/8"). You then need to figure out what diameter the plug needs to be, lay out your circle, and cut it out (jeweler's saw for hand work or a stationary jig or scroll saw* for powered work). Then glue it in, probably with Goop or a similar flexible adhesive.
*Nowadays, a lot of people use the term "jig saw" to mean a hand held tool; but it originally referred to something like this:
