This one, http://www.radioshack.com/product/in...ductId=3875403 set to 9v is probably the closest to what you need that RS sells these days.
I also came across this place http://www.retrogamecave.com/nec-ac_adapter.html which may be better since they're specifically making the adapters for use with the consoles but I don't know anything about them besides what their site says.
If you have any local game stores (not gamestop!) that sell vintage games, they might have Genesis adapters. Just make sure you get one for a model 1, the plug for a model 2 is different than what an NEC system would take.