Since some of you are already in the World of Ruin, here are the locations of the four new Espers, if anyone wants to know:
(SPOILER)
Sabotender (Cactuar?): Kill at least ten Cactuars in the desert south of Maranda. Step on the panel indicated here to fight Jabotender (Jumbo Cactuar, possibly?).

Gilgamesh: Bid on the Excalipoor item in the Jidoor Auction House. You may need to farm Gil south of Miranda for awhile, for the total cost will be 500,000 Gil (possibly as a joke since we could never beat that one old man before). Bet the Excalipoor in the Coliseum and fight an easy enemy there to fight Gilgamesh.

Leviathan: Ride the boat in Nikeah. I'm not sure if Locke needs to be in the party or not.

Diabolos: Clear the bonus dungeon, the Dragon's Den, and defeat Kaiser Dragon. This dungeon will only open up after you've defeated the eight dragons (and, by then, you may as well have beaten the main story too).
"I hear some fanatical members of the Cult of Kefka insist on spelling Kefka's name with Cs instead of Ks. That just seems silly to me. Kefka's Kefka. He's still the same villian, no matter how you spell his name."
!! xD