This page shows where you can find all the dress spheres from Final Fantasy X-2, and in what chapter.
Black Mage- After defeating the first boss(Boris) in the floating ruins at Mount Gagazet
White Mage- After defeating the boss(Flame Dragon) on Besaid Island(in chapter 1)
Gun Mage- Protect Tobli's Hypello cart(in chapter 1) at the Moonflow.
Alchemist- Clear out all the fiends at the Monster Arena in the Calm Lands(in chapter 2).
Lady Luck- Defeat Shinra at Sphere Break.(in chapter 3-5)
Berserker- Distroy 6 sets of fiends at Macalania Travel Agency.(in chapter 3)
Dark Knight- In a chest Under Bevelle.(in chapter 3)
Samurai- In the Kilika Temple, near The Chamber of the Fayth.(in chapter 3-5)
Trainer- If you gained trust from each Ronso, you can get it from Kimahri.(in chapter 3)
Mascot- Recived from Brother only if you have 100%.(in chapter 5)
Yuna
Floral Fallal- After defeating Logos, Ormi and a fem-goon on the Djose Highroad.
Rikku
Machina Maw- At the Oasis at Bikanel Desert.(chapter 2)
Paine
Full Throttle- At Macalania Forest, from Tromell. Speak with him 4 times.(in chapter 1 & 2)