Final Fantasy X/Equipment

Customizing
When Rikku joins your party you may customize weapons and armor as you see fit with any ability, as long as you have the necessary items. The equipment must have at least one empty slot in order to customize.

Once Rikku has joined your party, "Customize" will be added to the menu. Press psx: T to access the menu, select "Customize". Select the weapon or armor with an empty slot you want to add to. A list of available abilities you can add will appear. If an ability is not listed it is because you don't have enough items for that ability. Press psx: X to select the ability and confirm. The empty slot will then be replaced by the ability you've added. The only equipment you cannot customize are the Celestial Weapons (Even if they have an empty slot). To expand those, please refer to Celestial Weapons under Sidequests.

The best empty weapons and armor to customize can be bought from Wantz at the southern entrance of Macalania Forest. He can only be found after your party is deemed traitors and escape from Bevelle. And then only if you donated to O'aka. If you didn't donate to O'aka, or skipped talking to Wantz on your climb up Mt. Gagazet, Wantz will not be there. If Wantz isn't there, fighting Varuna and Adamantoise in Sin and the Omega Ruins will periodically drop empty 4-slotted equipment.