Vestments of Faith

Vestments of Faith is the tier 3 priest set.

Source
The set items can sometimes appear at the Black Market Auction House.

Most tier 3 items can be acquired through quests given by Father Inigo Montoy, who can be found in Light's Hope Chapel in the Eastern Plaguelands. Before the quests become active, Echoes of War must be completed.

The quests require several crafting materials, Wartorn Cloth Scraps (which drop from Trash Mobs in Naxxramas) as well as a token from bosses in Naxxramas.

The only exception is the, which drops directly from Kel'Thuzad in Naxxramas.