Robe of the Magi +6

Increases the Intelligence of the Hero by 6 when worn.

Source
Furion can find one in Brothers in Blood, chapter 5 of the Warcraft III night elf campaign, Eternity's End.

Maiev can find this in The Broken Isles, chapter 2 of the Frozen Throne sentinels campaign, Terror of the Tides.

Malfurion and Maiev can find one in The Ruins of Dalaran, chapter 7 of that campaign.

Illidan, Kael, Lady Vashj and Akama can find one in Lord of Outland, chapter 6 of the alliance campaign, Curse of the Blood Elves.