DO NOT use any generic three-in-one or two-in-one adaptors from eBay or Amazon, they're garbage and cause noise in the video signals and various other issues. Honestly, I would try to track down an official version by SEGA. A company named Retro-Bit sells a licensed version on their website., I can't personally speak to it's quality since it seems Retro-Bit products are hit-and-miss generally, but they do offer several other "officially licensed" Sega Genesis and Saturn products.
If you really want some high quality power, try RetroGameCave.com. They're a bit pricy, but they offer very high quality power supplies for several retro consoles, including the Model 1 Genesis.