Final Fantasy Tactics: The War of the Lions

You have to encounter human enemies in random fights when you're at higher levels; they'll have better armour and weapons equipped that you can then steal (or pilfer if you have Balthier) from them...

Most of the rarer equipment will never show up on enemies outside of story battles for theft, this is because they are coded that way. But many items can be obtained through other methods.

Ribbon - there is a 75% chance that after you poach a Wild Boar it will show up in the Poacher's Den for sale, if you're unlucky a Fallingstar Bag will be for sale instead.

Chantage - The common item for poaching Swine

Holy Lance - The common item for poaching Sehkret

Zwil Straightblade - The rare item for poaching Plague Horror

Lordly Robe - The only way to obtain it outside of battle is through Ad-Hoc mode. On the final Floor of the Brave Story rendevous mission you will encounter Delita, Valmafra, and 8 Female Darknights. Both Delita and Valmafra are wearing Lordly Robes, however you can only steal Valmaftra because Delita is also equipped with the safeguard skill. Beware losing your own equipment, four of the Dark Knights have steal as a secondary skill.

Chaos Blade & Chirijiriden - One copy of each can be obtained using the Treasure Hunter movement ability on the final floor of the optional dungeon Midlight's Deep. Extra Copies of the weapons can be obtained by exploiting high level Ninjas. Chirijiridens can potentially be thrown by ninjas at level 96+ and Chaos Blades can potentially be thrown at level 98+ (I say potentially due to the fact that if the ninja does not know the corresponding throw ability they will never throw the weapon). Consequently you can also obtain many other weapons this way as well including : Masamune(lv 95), Ragnarok(lv 97), Excalibur(lv 96), Save the Queen(lv 95), Defender(lv 92) and many more. Also notice that many of the weapons share level requirements eg. Chirijiriden and Excalibur, this means that if the ninja knows both the Throw Knight Sword and Throw Katana abilities he will never throw katanas this is because by default the ninja will always throw the weapon with higher Weapon Power.

Some items such as chantage and ribbon can be stolen off of unique characters such as Wiegraf or Meliadoul (before she joins your party).

Take a look at Qu_Marsh's FAQ. Ctrl+F this: "00rare" (without quotes). He gives you a list with all possible rare items and where/how to find them. It can be poaching, treasure hunting, stealing, or going into Melee/Rendezvous mode for more powerful exclusive gear.