[RipNC]

Easy way is a 120V to 12V (cigarette lighter plug) transformer.

I think my 7" display uses a 3.5mm x 1.35mm barrel jack, so you could also search for a 120v to 12V with that size barrel jack. Might be a good idea to check the amperage on the fuse in the 12v plug for your display and make sure your output isn't much more than that - probably 1 or 2 amps.