You need the Celestial Mirror. In order to get that, you need the cloudy mirror, then go to the place in macalania forest where you found the little boy who was lost from his parents, up on the shortcut road above the trees. Use the cloudy mirror in there, and it turns into the celestial mirror, which is also what you need to get all the celestial (ultimate) weapons.

You get the cloudy mirror at Remiem Temple, after you win the Chocobo race. To get to Remiem Temple, when in the calm lands, get on a chocobo, go up on the cliff by the exit to Macalania forest. Go past the exit, and you'll see a chocobo feather on the ground. press x at it, and you'll fly down to another ledge, with a pass through the mountains. Go through to get to Remiem temple.