Ok, to get Yuna's Celestial Weapon, the Nirvana, you need all 10 fiends from the Calm Lands. Open the chest that appears there with the Celestial Mirror, to get the Nirvana.
The Moon Crest, is in a cove at Besaid Beach.
The Moon Sigil, is obtained by beating ALL of Belgemine's Aeons at Remiem Temple (with the possible exception of Magus Sisters, I forget).
Beat her Aeons up to Bahamut, and you get the Flower Sceptre, I believe, and the Blossom Crown from collecting all fiends on Mt. Gagazet, and you can now unlock the Magus Sisters as well.