Pendant of Energy

Increases the mana capacity of the Hero by 150 when worn.

Source
Arthas can find one in Digging up the Dead, chapter 2 of the Warcraft III undead campaign, Path of the Damned.

Thrall can find one in The Long March, chapter 2 of the orc campaign, The Invasion of Kalimdor.

Thrall and Cairne Bloodhoof can find one in By Demons Be Driven, chapter 8 of that campaign.

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

Kael and Lady Vashj can find one in A Dark Covenant, chapter 2 of the alliance campaign, Curse of the Blood Elves.

Kael, Lady Vashj and Illidan can find one in Gates of the Abyss, chapter 5 of that campaign.