I've got the big metal ones, but since I always have to cut the rheostat shaft to replace the faceplate I can't salvage the small knob. I just give it back to the customer and let him try and drill out the set screw.
Jared at Stumpfs has a huge graveyard of these machines. And the pics he sent me show evry machine having metal knobs, so I asked him what he'd take for a dozen sets, and he said they weren't available. I'm looking everywhere I can for them.