you need 30 X wings to discovery. to get these you can either:

1. bribe malboros in the calm lands, i think it's 645,000 gil and you can get a max of 4 each time. malboros are found behind the travel agency in the calm lands

2. fight shinryu in the monster arenas. if u overkill it you will get 2

3. if memory serves me i think u can get 30 by getting so many chests in the chocobo race under remiem temple. you may need to look into this a little more cos i don't have my guide with me right now