Usually all these multi adaptors with exchangeable plugs should work for the sgfx.
Yeah, that's what I use. A little googling reveals:
"The official NEC PSU for the Super Grafx was the model # PAD-113,
its specs were as follows: Input AC100V 50/60Hz 16VA Output DC9V
800mA (negative tip)."
A 6.3mm x 3.0mm pin (Radio Shack "Q") should fit this.