There are a total of 10 Chaos Chests. Chaos Key is dropped by Key Orbling in any trials of elements.
Chaos Chest #1
Item: Mirage Band
Location: Midi Toy Box - Secret Room, just teleport to "Vence City of Water -> Hidden Room".
Chaos Chest #2
Item: Ring of Eluned
Location: Midi Toy Box - Secret Room, just teleport to "Vence City of Water -> Hidden Room".
Chaos Chest #3
Item: SP Capsule x3
Location: Vibra - Underground Jail. Teleport to "Vibra Royal Capital -> Underground Jail".
Chaos Chest #4
Item: Photon Bullet
Location: Vibra - Underground Jail. Teleport to "Vibra Royal Capital -> Underground Jail".
Chaos Chest #5
Item: Angel Rod
Location: Vibra Royal Capital - Main Street.
Chaos Chest #6
Item: Ouroboros
Location: Vibra - Town Center, Leo's House, backyard.
Chaos Chest #7
Item: Graviton Knuckles
Location: Vibra - Town Center, Leo's House, backyard.
Chaos Chest #8
Item: Soma Blade
Location: Vibra - Town Center, Leo's House, backyard.
Chaos Chest #9
Item: Grace Globe
Location: Hidden Valley - Bernard's Laboratory, F1.
Chaos Chest #10
Item: Crystal Quartz
Location: Hidden Valley - Bernard's Laboratory, F2.