I find it in the same place it's been for a long time now. I use the actual Xbox 360 to make purchases, since I've had issues buying from the Xbox Live website.
Right bumper to the Games Screen from the Xbox Home page, and then choose browse games.
Right Bumpr to the add-ons page and choose A-Z.
Choose D, and then right bumper until you see Dead Space and choose it.
Right Bumper to extras, and choose See All.
They should all be there.
XBL: Master Zoen
No unfortunately, it would make me want to play through again if I could get them on PC.
Only Dead Space 2 and 3 have DLC on the PC. For whatever reason, EA left the original Dead Space "dead in the water" in regards to dlc.
An official patch came out a few years back that gave PC gamers access to the 3 DLC packs of Dead Space 2 for free. It seriously cheesed off the console gamers who had to buy them for $5 apiece. For that matter, they still cost $5 each on the consoles. EA could at least make them free now.
XBL: Master Zoen
Ok, thank you for the answers :)