I lost my disk 1 to GTA 5 on Xbox 360. I still have disk 2 but I lost all downloadable content off the xbox's hard drive. What can I do to recover my data?
I suggest going over to Rockstar Games support page, and submitting a ticket asking if you can buy a replacement disc.
I have no clue what Rockstar Games support system or policies are like, but I have heard countless stories of many gamers contacting support and getting assistance replacing missing game peripherals and game discs.
Publishers won't advertise that you can call them up and order individual game discs, but many offer this service because they understand that if you scratch a disc in a 2-3 disc game, most players would want to find a cheaper way to replace the disc than getting an entirely new copy. Plus, buying from them puts a few more bucks in their pockets, rather than the second-hand market.
Give it a shot. It can't hurt, and I'd be interested in getting a comment below on what they say. I'll try to update this answer, depending on how it turns out for you.