Super Grafx uses DC 9V 800mA. Polarity of plug is outside positive, inside negative.
I don't know what size plug though, I would guess 2.1mm.
Should be easy to track down a cheap, suitable, power supply. The one I link to above will work fine as long as the Super Grafx has a 2.1mm plug. If not, get one of those adjustable power supplies with interchangeable tips.