They sell license plate mounting bolts that have built-in LED lights, but I don't worry about not having a license plate light. It's required by law here, but it's not an item they check during a "safety inspection."
I'm not one to break laws, but after the first few times a police cruiser was behind me at night and I was wondering if they were going to pull me over and they didn't, I stopped worrying about it. I really think they just don't care and, unlike a larger car plate, by the time they get close enough to actually read the numbers on the smaller m/c plate, it's definitely going to be in the full glare of their headlights.
But, if I were to feel inclined to illuminate it, I'd probably just hit a junkyard and get a license plate light off a car and wire it in.