Aha. I really should've known that; it's even mentioned on this very page, and I've used salt quasielementals before. Sorry for that, just poor reading on my part.
Anyway, if we avoid planes outside the 3e cosmology, then I suppose it's just process of elimination. Fire's out because it would kill them. Water works thematically but physically it would be fairly ludicrous. That leaves Air and Earth, either of which I could see an argument for, though personally I'd lean more towards Earth since ash quasielementals feel more solid to me. Shapeshifting, yes, but still solid. So use one of those two planes, with a mention in the description line that, like you suggested, they tend to dwell in negative-dominant areas of whichever plane is chosen, and near to the Plane of Fire so they can feed on its warmth safely?