Halo 4 has a lot of nice armor customization options, including the Deadeye Helmet.
The Deadeye Helmet was only available if you pre-ordered Halo 4 from Play.com or Best Buy though.
It may be available as DLC at some point, but it’s currently exclusive to pre-orders only.
You should be able to find someone selling their Deadeye DLC code on Ebay though. As of writing this article I found a ton of them for sale.
Check out my other guide for the complete list of unlockable armor sets in Halo 4.