The tuning knob appears to be a standard 1/4 inch shaft with a flat. It's one of the most common shaft sizes.
Finding a knob that will fit would probably not be that hard if you don't need the original knobs looks. The knob on eBay seems overpriced but maybe not for someone wanting to restore a VR-5000 to the factory look. If you have any old radios or boomboxes or stereo receivers laying around, there's a good chance you may find a knob that will work.