Actually I just remembered I had a multimeter in my parts box and I did a test on my X'Eye power supply tip - it's outputting 14.3 volts! So I can probably just get a 12 volt generic power supply and that will be fine.

I did some other checks:

Mega Drive model 1 AC adapter:
Stated 10v
Actual 14v

Master System AC adapter:
Stated 9v
Actual 14v

US SNES AC adapter:
Stated 10v
Actual 18.5v

Mega Drive model 2 AC
Stated 10v
Actual 13v