Mantle of Intelligence +3

Increases the Intelligence of the Hero by 3 when worn.

Source
Arthas and Jaina find one in Ravages of the Plague, chapter 3 of the Warcraft III human campaign, The Scourge of Lordaeron.

Arthas and Kel'Thuzad can also find one in The Siege of Dalaran, chapter 7 of the undead campaign, Path of the Damned.

Thrall can find one in The Long March, and one in The Oracle, chapters 2 and 7 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Daughters of the Moon and one in Brothers in Blood, chapters 2 and 5 of the night elf campaign, Eternity's End.

Maiev can find one in The Tomb of Sargeras, and one in Wrath of the Betrayer, chapters 3 and 4 of the Frozen Throne sentinels campaign, Terror of the Tides.

Arthas, Kel'Thuzad and Sylvanas can find one in King Arthas, chapter 1 of the scourge campaign, Legacy of the Damned.