What is the needed resistance value? Remove one of them from the board, and measure the value of the element from end to end. No connection to the wiper needed for this test.
The pots don't look like anything special, just standard cheapo trimpots. Should be able to get something similar from most any parts supplier. If you are a real stickler for tweaking the color balance, you might want to swap in multi-turn trimpots for finer adjustment.
