Essentially you’re after a potentiometer? Most electrical suppliers will sell them, you just need to work out the resistance required
LEDs need around 20mA. This will fry the track on most pots.
To reduce the brightness you need to reduce the current. Two ways to this, either keep the same voltage but introduce more resistance, or keep the same resistance, but reduce the supply voltage.
A forward only ESC will combine both, can be worked from a spare channel or a servo tester.
Rotary switches (used to be called Yaxley Switches way back) could be got in various combinations of 12 - 3 pole 4 position, 4 pole 3 position etc. Not having needed one for years, I would need to look again. But a cheap Forward only ESC plus maybe a servo tester is probably the simplest and cheapest way to do it. Might need a voltage dropper to supply off the 12 volts, though, depending on the ESC.