I couldn't find Dead Money (360) on Amazon so I was wondering if PC DLC will only be available through Steam.

Bear in mind, the Fallout 3 DLC that came on disc were in pairs. So you're not going to see Dead Money in stores UNTIL the second DLC is out.

downloadable content...

That being said, there is a precedent for DLC coming out on disk, but is usually well after release and is usually packaged with other dlc or the whole game. So if F03 is anything to go by, it will be availlable on a disk with FNV with al lthe dlc thats released for it in about a year or so.

Dead Money is an Xbox Live exclusive right now. Why would it be on Amazon?

As a potential pre-order I would assume.

Okay, but not till it's officially announced at least.