When I ran into this situation, I needed 3 of them. HD Graphics Genesis, SMS and 32x all use the same adapter. Generic adapters as mentioned were not in my game plan, as one I had used popped the voltage regulator in the SMS. Radio Shacks still existed at this time, so replacing that part was easy. But since then, I do not trust anything but the official adapter.
I had resorted to buying Model 1 Genesis system lots on Ebay just to get the power adapters, as buying the adapter on its own seemed to be more expensive.