Look into
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Parts-Express,
MojoTone, eBay and other electronics supply and/or hardware sources. Most all will be carrying Penn-Elcom, even if they are with private labels. Find the one that most closely matches the size of the feet you do have and check the size and style of the screws used on the other feet. You may or may not find the screws needed. I've built a few amps and cabinets and the suppliers I listed seem to be the most readily available and decent selections, AES being my number one source.
My Rumble 200 didn't come with any feet. I contacted Fender, but they didn't have anything listed and just recommended I get something similar from other sources. I got a set the seemed to be approximately the same size and just got some small sheet metal screws that appeared to be the appropriate siz
I'll second AES. When I was building/modding/repairing amps, I bought a lot from them. I don't think I used the other two sources but have heard only good about them.