wings to discovery, which you can get by bribing:
Marlboro - Wings to Discovery x4 for 540,000 gil (Calm Lands or Cavern
of the Stolen Fayth)
Great Marlboro - Wings to Discovery x8 for 1,280,000 gil (Sin or Omega
Ruins)
or get 1 or 2 for defeating shinryu at the monster arena!