Rainmaker

Rainmaker is a Blue Mage's arm.

Source
Rainmaker can be purchased from vendors or obtained as a quest reward.

Vendors
Rainmaker can be purchased for 1,000 from the following vendors:
 * Calamity Salvager - Limsa Lominsa Upper Decks
 * Calamity Salvager - Old Gridania
 * Calamity Salvager - Ul'dah - Steps of Thal

Quests
Rainmaker can be obtained as a reward from the following quest:
 * Out of the Blue