They're not made by anyone, but sometimes you can find that part at an RV salvage or used parts place. Try http://www.rvdoctorgeorge.com
as a start. There have been other aquamagic 3 owners who found the Thetford parts there.
If all else fails and you can't find the knob, you could always replace the toilet with an aquamagic 4 for around a hundred bucks. It's using a shotgun on a gnat, but there would be no issue with parts if it ever breaks.