Slippers of Agility +3

Increases the Agility of the Hero by 3 when worn.

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

Maiev can find one in Wrath of the Betrayer, chapter 4 of the Frozen Throne sentinels campaign, Terror of the Tides.

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