When you’re searching for Halo 4 DLC on your local eBay site, make sure to check the “Worldwide” box for the best results.
Most sellers are willing to send the codes via email or eBay message in addition to posting the physical cards as per eBay requirements.
I’ve gotten most of the preorder DLCs from eBay via this method.
EDIT: Could a mod sticky this please? It’s very useful information.