Fully customizable gear?

[Tavrobel]

He only appears if you talk to him on Gagazet, which is rather dumb if you don't, because he sells insanely useful items in that desolate land

He will tell you that O'aka was arrested for aiding Yuna. After you enter Sin, he will appear at the Thunder Plains entrance of Macalania Woods. (Go by airship, and go south for 3 seconds).

4 slot items are dropped randomly by Varunas, Adamantoises, and Master Tonberries, approximately 10% of the time. These armors and weapons are also dropped randomly by Area and Species Creations, as well as most of the Originals, (closer to 5%).

The Absolutely BEST armor you can possibly have is
BHPL
BMPL
Ribbon
Auto-(?, anything)

Your choice in final slot. If you have the NA version, replace Ribbon with another Auto. I use Auto-Shell+Auto-Protect, or Auto-Med/Regen. Auto-Pheonix is useless when you get 255 defense/MDef/agility and enough HP to withstand Nemesis' Ultra Spark, because even if you DO die, by that point, you can cast Auto-Life, or one person will live and he can cast Full-Life, which is better than a Pheonix Down, unless you have 99 Mega-Pheonixes and no Pheonix Downs.

Originals are the primary source for higher end Armor synthesis items, as well as Dark Matters. Area and Species Creations are also a good source, who have a slightly worse drop rate for Dark Matters than Originals.

Original: 25% chance for dropping Dark Matter, 1 for kill, 2 for Overkill
Area/Species: 12.5% chance for dropping Dark Matter, including those who drop more than 2 items at death (chances will drop to 6.25%, 1/16).

Auto-means that the ability is always set at the beginning of the battle, and cannot be dispelled-ish.

Armor Abilites:
Auto-Shell -- casts Shell at outset of battle, cannot be dispelled
Auto-Protect -- casts Protect at outset of battle, cannot be dispelled
Auto-Haste -- casts Haste at outset of battle, cannot be dispelled
Auto-Regen -- casts Regen at outset of battle, cannot be dispelled
Auto-Reflect -- casts Reflect at outset of battle, cannot be dispelled

Auto-Pheonix -- when ally dies, Pheonix Down/Mega-Pheonix is used on character, requires at least one surviving party member that is not already Confused, Berserked, Petrified, Asleep, or if all members are dead

Auto-Med -- character uses Remedy/appropriate curative item for an ailment on self, Remedy automatically used if no specific item is findable, or if character suffers multiple ailements (including Zombie/Curse)
Darkness -- Eyedrops, Remedy for multiple conditions
Silence -- Echo Screen, Remedy for multiple conditions
Poison -- Antidote, Remedy for multiple conditions
Zombie/Curse -- Holy Water, Remedy will cure neither, Curse/Zombie have lower priorities than other ailments
Confusion/Berserk/Asleep/Petrified -- cannot act logically, therefore, none
Death -- requires Auto-Pheonix, or Life spells
Doom -- cannot be dispelled, causes Death at end of count

All ailments leave character when battle ends, including Death, at which case, character is revived with 1 HP after battle.

Where items that customize higher abilities are found are also contained in the FFX strategy guide, pg. 50/51 for abilities, pg. 172-174 for item drops at Arena. Beastiary contains normal encounter rates, Arena has slightly better Bribe and item drops. NA version only.

If you have a specific item/ability, PM and I can give more details.