What PSU do I need for a Sega Megadrive/Genesis?

The power supply depends on the model.

Wikipedia gives the connector types for two of these models:

The power input varies depending on the model - a model 1 uses a 2.1mm barrel connector with a negative tip, and requires 9-10 volts DC at 1.2 A. The model 2 uses a EIAJ-03 connector with a positive tip, and requires 9-10 volts DC at 0.85 A .

So, going back to the chart:

• The MK-1602(-1): for use with SEGA Genesis 1, CD 1, CD 2 uses a 2.1mm barrel connector
• The MK-2103(-1): for use with SEGA Genesis 2, 32X uses a EIAJ-03 connector with a positive tip
• The MK-1479: for use with SEGA Genesis 3 appears to use the same connector as the MK-2103(-1), but I cannot tell. (I can't find a good photo showing the connector either)

As mentioned on the GameTrog page, be sure to match the voltage if you decide to go with a 3rd party PSU and be sure it can deliver at least the amount of current specified for your model. An incorrect power supply can damage your system and potentially yourself. Overall, I recommend finding an official PSU.