iTunes, but if you end up liking what you read don't expect to buy the full thing off of Apple's virtual store shelf. The graphic novel is a prologue to events in the game and is "exclusive" to the Fallout: New Vegas Collector's Edition.
The un-interactive text adventure is published by Dark Horse Comics and written by New Vegas' senior designer, Chris Avellone. The comic also apparently gives "clues to in-game missions for the sharp-eyed reader." Now we totally want Where's Waldo: Fallout Edition.