Sobi Mask

Increases the Hero's rate of mana regeneration by 50% 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 can also find one in Blackrock & Roll, Too!, chapter 6 of the undead campaign, Path of the Damned.

Thrall can find one in The Oracle, chapter 7 of the orc campaign, The Invasion of Kalimdor.

Tyrande and Furion can find one in The Druids Arise, chapter 4 of the night elf campaign, Eternity's End.

Kael and Lady Vashj can buy these from Goblin Merchants in The Search for Illidan, chapter 4 of the Frozen Throne alliance campaign, Curse of the Blood Elves.