[AndrewJ]

If you only want a simple manual PWM unit, you can make one yourself pretty simply using an LM324 chip and a few resistors/capacitors, and then select a MOSFET that will handle the power you want.
3 channels of the LM324 are used to make a sawtooth oscillator and the fourth channel becomes a comparator that compares the sawtooth to the potentiometer output to drive the MOSFET.
I have made quite a few this way and it only takes a few components.
The single channel one ( ref piccy ) can be made with a single LM324,
but if you add another LM324, you can make a 5 channel unit for virtually no extra work.
( That said, its still probably cheaper to buy something from China and repurpose it, but if you make it yourself, you can set the PWM freq and choose your own mosfets etc )

Andrew