Help with a Tightening Knob question
Before you go any further, you should take either the broken knob or the flash (which ever is easier) to your local ACE hardware and ask them to help you identify what the actual thread size and pitch are.
They should be able to do that for you in a couple of minutes. Once you know the size (be it metric, UNC or UNF) you can better locate a proper replacement. FWIW, ACE carries a small line of threaded knobs themselves (at least my local ACE has some) so you might just be able to find the solution while you are there.