Ring of Protection +2

Ring of Protection +2 increases the armor of the Hero by 2 when worn.

Source
Arthas and Jaina find one in The Cult of the Damned, chapter 4 of the Warcraft III human campaign, The Scourge of Lordaeron.

Arthas can find one in Trudging through the Ashes, and one in Into the Realm Eternal, chapters 1 and 3 of the undead campaign, Path of the Damned.

Tyrande can find one in Daughters of the Moon, chapter 2 of the night elf campaign, Eternity's End.

Tyrande and Furion can also find one in The Druids Arise, chapter 4 of that campaign.

Maiev can find this in The Tomb of Sargeras, chapter 3 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael and Lady Vashj can find one in The Crossing, the secret level of the alliance campaign, Curse of the Blood Elves.