Since the current draw of an LED or bulb would be minimal, you have many options. I'd hook it to the 12V supply going to the dash instruments if a bulb, and the 5V supply coming off the voltage regulator for the dash instruments if an LED.
If using an LED, remember to put a resistor in line with it, or the LED will burn brightly, but briefly. The resistor value depends on the LED specs and the supply voltage. Some panel mount LEDs have a resistor built in.
I considered adding an ignition light, but thought that the instruments coming alive when the ignition was turned on was good enough.
Finding panel lights ready-to-go seems to be rather difficult. The auto electric supply web sites don't have them. I finally found ones on the shelf at Shuck's for about $4 each.