I'm assuming you're talking about the normal one and not the SP, since I think the SP came with the wall plugin adapter. In which case yes there are some adapters like you're looking for, though I don't know if any are Nintendo made. I had a decent 3rd party rechargeable battery for mine that you could plug in while playing.
As for other add ons, you're REALLY going to want some kind of snap on light. The lack of a back light can be pretty brutal depending on the game you're playing. I almost never played mine without using a light.
But if you were getting the SP then it should have the power cord you wanted with it, plus has a backlight, so there's really nothing you need for that one.