5mA to 40mA current panel mount indicator LEDs
In that case:
-resistor
OR
- LM317, the cheap to220 version suffices
In both cases bring the voltage down to about 2-3V more than the Led Vf to minimize the powerloss.
To achieve that use a switch mode voltage regulator such as the cheap ones on Ebay.
On/off control could be a small P-fet switching the voltage to the resistor or lm317.