Feathered Harpoon

Lancer's Arm, item level 12. Equip level 12, LNC DRG.

From 44 Courage of Stone and 1 more quest.

From quests

44 Courage of StoneReward
59 A Hearer Is Often LateChoose one of 4

Other databases

Eorzea Database (the official one, on The Lodestone)
Console Games Wiki
Garland Tools (vendors, crafting, drops)