Marathon Emblem Collector is the most complete source for Marathon emblems: what they look like, exactly how to get them, and whether they're still available.
MEC currently tracks 57 Marathon emblems and 50 backgrounds
Welcome to the MEC alpha (v0.09.01.12)! Open TODOs:
-Fix home page background preview icons - DONE!
-Add backgrounds to filters / menus - DONE!
-Fix Category dropdown - DONE!
-Add background cosmetics - DONE!
-Add background cosmetic in-game descriptions - DONE!
-Verify background cosmetic data - DONE!
-Add backgrounds to search page - DONE!
-Fix mobile views across the site (again) - DONE!