As far as I know you can't bet anything at the Colloseum to get Imp's Armor. I know you can Steal them from Tyranosaurs, but I don't remember what other enemies might give you them.

Btw, for those talking about being unbeatable: try equipping Terra, Locke, Edgar, or Celes with the Illumina, Paladin Shield, Bard's Hat, Force Armor, Beads, and Zephyr Cape. That'll boost the character's Evade% by 120 and Magic Block% by 140. To say the least, a lot of stuff will miss