Stupid question, I know, but: are you sure it has broken? Not just fallen off? The camera looks like one of the newer F models. I have a donor C220 F with a lower knob intact (upper has been taken off for another camera where the knob had fallen off.) If you think you can use that, drop me a line. But if you are living in the US, check www.keh.com first.
My impression is that these plastic knobs are a design failure . they seem to shear off and break quite easily.