Rob may have spare knobs, but they are otherwise hard to come by. People who did the illumination hack may have spares that would work, but it was never an aftermarket item.

I might have a spare Mark2A knob in my box of parts. Let me check tonight...

By the way, not to try to talk you out of it, but make sure that you are very comfortable with installing the encoder yourself before trying to do it on your own. Those that have tried it said that it is extremely tricky to do, and there are few (if any) replacement parts for the display board or encoder daughter board.