Does anyone know where I can get threaded 20mm retaining clips to hold in my driver’s and PCBs
From the manufacturer. Depends on the thread that is used.
Your question seems a bit eh… universal.
Please specify brand and model of flashlight you want a retaining ring for.
Fasttech used to sell them (but does not re-stock), Kaidomain still does.
If all else fails, ask the manufacturer.