Killing Fallen Majors and Fallen Mages is a good way to get Garlaige Coffer keys. Back at level 53 I got into a Garlaige EXP Party (MNK/WAR, MNK/WAR, BRD/WHM, BLM/WHM, WAR/NIN, RDM/WHM (myself), it was the best party I was ever in), and we got 6-8 keys through 4 hours of partying. We got a key for everyone in our party, and we even swapped members in and out so other players could get a free key. I'd be willing to help you, but we need to open the Banishing Gates for me too, I want my Warlock's Tights.