If I remember right, that knob attaches to a 1/4" shaft, you should be able to find a knob at the local hardware store that will work, although it might not be as pretty as the ones that Beseler will sell you.
As an aside - there is a local college which has a darkroom with Beseler 23CII enlargers in it, some set up for 35mm, some for MF. I thought that I could use one of the 35mm ones for MF by bringing in my own negative carrier and lens (I have a 45MX at home, so same lensboard, and I happen to have a 6x6 holder for the 23C) - when I got to the darkroom, I discovered that the knob for adjusting the upper bellows had been removed from all of the enlargers to keep students from messing with it.