Thundercloud

Thundercloud is a Lancer's Arm.

Source
Thundercloud can be obtained by exchanging 5 with the following vendors:
 * Aelina - Mor Dhona
 * Expedition Liaison - Eureka Pagos

Thundercloud can also be obtained during the Moogle Treasure Trove event by exchanging 30 with the following vendors:
 * Itinerant Moogle