In Laguna's dream sequence at Lunatic Pandora, go straight until you reach a circular room with a ladder at the center. Go down and you will find that there is a key somehwere. I have no idea what it does so don't ask me. Tehn, go to a place where you see three doors on the floor. Tamper any lock possible. There will be one where Laguna can't open. One of them is already locked shut so you won't hear the clicking voice. Then, blast the rocks. It will open the door Laguna didn't manage to open earlier. However, you won't be going back.
Come back as Zell(earliest) or Squall(latest) and explore Lunatic Pandora thoroughly before doing anything else. The room where you opened the lock will now allow you to enter. There is a Power Generator there. Walk down the hall and you find yourself at the place where Laguna dropped one of the keys. There is a draw point for Ultima and Combat King 005 magazine. If you don't want to fight, simply equip Enc-none.