I get a lot of requests asking for kits to convert computers into USB keyboards. I normally only sell the converted units, but I've started rolling these outs as kits. The Spectrum and Spectrum+ kits complete with custom keyboard membranes are already available, and this is the version for Commodore keyboards.
Commodore USB keyboard kits. Or, if you don't want to convert the Commodore yourself, you can send it to me to be converted, or buy one of the already converted units from my Etsy store.
Update: Available new, Commodore 64 USB keyboard and dual joystick boards.
Update: The Time Shared Commodore 64 USB Keyboard kit is here