Ring of Protection +1

Increases the armor of the Hero by 1 when worn.

Source
Thrall finds one in Chasing Visions, chapter 1 of the Warcraft III prologue campaign, Exodus of the Horde, by defeating a golem.

Arthas receives one as a reward for the quest Save Timmy in the human campaign, The Scourge of Lordaeron.

Thrall can find one in Landfall, chapter 1 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Enemies at the Gate, chapter 1 of the night elf campaign, Eternity's End.

Maiev can find one in Rise of the Naga, chapter 1 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael can find one in Misconceptions, chapter 1 of the alliance campaign, Curse of the Blood Elves.

Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of the that campaign.

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