Hi Timothy,
That knob looks like it has a very standard "D" fitting. I'd suggest taking the heater to a hardware store where they'll have a selection of knobs available. It should be easy to find one that will fit; it just has to be smaller in diameter than the opening in the heater. If you're not concerned about actually matching the existing it won't be hard, or I suppose you could just get two matching knobs and replace them both.