I recently bought a bare Genesis 2 deck as well (absolutely flawless cosmetic condition, which is the main reason I went ahead and got it). The best deal I could find on eBay for accessories was from a seller named gameliquidation. He sells AC adapters, stereo AV cables, and control pads individually, but it's better to buy them bundled. Usually he seems to sell an AC adapter, stereo AV cable, and two six-button pads for $14.95 and $19.95 BIN. Hardly anyone bids on them so you should be able to get them for $14.95. Shipping was $8.25. Not sure if you'd feel ripped off by that or not. He ships them stuffed into a padded envelope first class mail, which isn't the greatest, but it's unlikely the goods will be damaged (not like a fragile cardboard game box or jewel case).
The AC adapter was Performance brand, the AV cable was Recoton, and the control pads were Sega branded (but they looked like the Majesco rereleases of the pads, not the original Sega ones). At the time, that was the best deal I could find for those accessories on eBay, including shipping.