According the
Ninja 250R 2009 parts catalog, the standard bulb for the license frame is part 92069A.
Don's Bulbs lists this as 92069-1007, 3.4W 14V incandescent type 158 bulb. So go look for a 158 equivalent at your local parts store, or order online. If you want to go LED, my favorite site is Super Bright LEDs, who have a
drop-in 158 replacement (WLED Miniature Wedge Base Bulb) in many colors. They have built-in diodes and current-limiting resistors (LEDs need a resistor not only to provide a load to flasher relays, but to keep from frying the LED).
Hope this helps!
EDIT: This assumes you're re-using parts of the fender or whatever. You could always go with a completely new frame with built-in LEDs or whatever and just mount it and hook up the two wires.