1.- Buy them off gem store (the most common)
2.- You get one as a reward from personal story at level 10, 30, 31 and 50, according to wiki. Some people have gotten more keys by creating a new char, going trough personal story until they get the key, delete, repeat. It hasn’t been confirmed as it being or not an exploit by Anet so far, but they haven’t taken action against it either, probably because there’s nothing game breaking that can be found inside chests, at least outside of RNG skin events.
3.- very Very VERY VEEEERY rare drop from any mob in game. I’ve been playing since release and I’ve gotten 3 keys as drops, and from what I’ve read from friends, I can count myself lucky to have gotten that many. No idea whether magic find affects their drop chance, though.
4.- Rare rewards from map completion and daily achievements (I’m yet to see one from dailies, though, but I know people that have)
NOT ANYMORE:
After the halloween event where the central statue in Lion’s Arch was destroyed by Mad King Thorn, there was an event where the lionguard was receiving donations of groups of 3 pieces of gear of the same quality + a donation package (bought for about 1s) to raise materials and resources to rebuild the statue, and in exchange gave you commendations, you could exchange 285 commendations for 1 key until the christmas 2012 event. Might or might not return for next halloween, currently there’s no use for those commendations other than saving them in hopes that the event will return this year.
It’s a pile of Elonian protection magic, mixed with a little monk training,
wrapped up in some crazy ritualist hoo-ha from Cantha.
A real grab bag of ‘you can’t hurt me. They’re called Guardians.
The keys very, very rarely drop as loot, but they do. Personal story is the most common way of obtaining them, other than using gems (either bought with cash, or traded for in-game gold) to buy them from the cash shop.
