Decided to take a shot with these reproduction knee pads for a CB350 fuel tank. Pricey, and they took almost forever to arrive, but they seem quite decent. I can't attest to the quality of the rubber, but time will tell. However, they fit perfectly and are nicely flexible. Much easier to put on than the old stiff rubber of the originals.

Honda CB250 CB250K0 CB350 K0 K2 K3 Rubber Tank Knee Grip LH&RH // A PAIR | eBay