Boots of Elvenkind +6

Increases the Agility of the Hero by 6 when worn.

Source
Arthas and Kel'Thuzad find one in The Siege of Dalaran, chapter 7 of the Warcraft III undead campaign, Path of the Damned.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of the night elf campaign, Eternity's End.