Pendant of Mana

Increases the mana capacity of the Hero by 250 when worn.

Source
Arthas and Muradin Bronzebeard can find one in Dissension, chapter 8 of the Warcraft III human campaign, The Scourge of Lordaeron.

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

Thrall and Cairne Bloodhoof can find one in Where Wyverns Dare, chapter 6 of the orc campaign, The Invasion of Kalimdor.

Illidan can find one in The Brothers Stormrage, chapter 8 of the Frozen Throne sentinels campaign, Terror of the Tides.