Special Dress Spheres are powerful Dress Spheres that you can equip after you've already had on all the dresses in the garment grid in a battle first, then pressing L1. This makes the Dress Sphere switch to the special dressphere. Here's how to get these dresspheres.

Yuna
In chapter 2 when you look for the three Leblanc Syndicate uniforms. Go to Djose Temple and head south on the Djose Highroad. On the road there's a sphere, which is Yuna's special dressphere.

Rikku
In chapter 2 go to Bikanel Desert and Nhadala will say that she thinks something strange is happening near the Oasis. Talk to the pilot twice to go to the Oasis and there's a sphere lying there which is Rikku's special dressphere.

Paine
It's possible to get Paine's special dressphere in chapter 1 and in chapter 2. In chapter 1 go to Macalania Woods and to the place where you fought the Spherimorph in Final Fantasy X. Talk to Tromell four times and he'll give it to you. In chapter 2 go to Macalania Woods and to the place where Yuna and Tidus had their kiss scene in Final Fantas X. Talk to the musician and he'll tell you to find his friends. After you've done that go to the place where you fought the Spherimorph in FFX and Tromell will be there. Talk to him four times and he'll give you the special dressphere.