They are potentiometers, or variable resistors. I don't think you can just buy a knob, because they are proprietary. You'll need to know the range of the pot, by holding a multimeter to the two sides and adjusting it to Rmax. That will tell you the replacement you need.
That said, I don't think the missing knob should be affecting the circuit. It is just there to help you adjust the pot with a screwdriver.