Didn't buy game at release :( Think they may make it available for season pass holders later or something?
You may be able to find a game at gamestop that still has the code in it. My buddy was a manager there. He gave me codes for lots of games. You'd be amazed at how many people never used their 48hour codes or in game codes. You'd probably have to go to a rural store though for the best chances. A lot of times you can ask the manager if such codes are still in it.
Other than that I guess you could try Ebay.
If there are two evils, I'll try the one I haven't done yet. Gamer tag: Spinal101