Halo 4 has a lot of customization options now, and the Locus Helmet skin seems to be on the popular side.
Unfortunately the Locus Helmet skin was only available if you pre-ordered Halo 4 from the Mighty Ape website.
Hopefully it will go on sale via DLC at some point, but it’s currently exclusive to pre-orders only.
You may be able to find someone willing to part with their Locus Helmet code on Ebay though. At the time of writing this, there are a lot of them currently for sale.
Check out my other guide for the complete list of unlockable armor sets in Halo 4.