Referring to http://ffxi-atlas.com , there is a large amount of key dropping Yagudo in basement 1. I suggest using this guide to find the chest(turn on chest locations, they are blue squares on the map) and area to kill them. Along the way to basement 1 there is a few to kill as well, but not many. I suggest going there as it might take you hundreds of kills.
As for my luck getting the key, it took me only 33 kills to get this key as PLD67/THF25, for my bst af boots. It was rather easy. I didn't have to rest at all thanks to auto-refresh. I also got various song scrolls(sold to vendor for 200-800g each, Auction not much better), and doton:ni which I sold to the auction house for 90k. All this in less than a hour. Yeah, I'm lucky. Note:I didn't open the chest with pld, because I wasn't completely sure if I would get the bst item. I came back after as BST.
im a 59 blm. i was with a 62 war. we killed 4 mobs that drops this key. we got 2 drops out of the 4. i think this is the easyest drop for keys. but it could be diff for others.
this was happening when i was doing PLD AF2.i was with this 75mnk and i was fishing for that NM fish and when my freind was resting in the pond a slime was attacking him and i was gonna join the battle and when i was scanning to target the monster i seen that in the coner was a chest and i got my hopes up very bad and all i got was a 30k electrum hairpin :/
This is different from the Oztroja COFFER key which opens the COFFER chest which may contain an astral ring. The TREASURE chest key will open a TREASURE chest which may contain an electrum hair pin or various gil amounts...at least those are the 2 things people have confirmed for me in the TREASURE chest, there may be other things in it but NOT an astral ring.