Frostfire Regalia

Frostfire Regalia is the Tier 3 set for Mages.

Source
Most Tier 3 items can be acquired through quests given by Archmage Angela Dosantos, who can be found in Light's Hope Chapel in the Eastern Plaguelands. Before the quests become active, must be completed.

The quests require several crafting materials, s (which drop from Trash Mobs in Naxxramas) as well as a token from a boss in Naxxramas

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

Patches and hotfixes
As of the launch of Wrath of the Lich King expansion and the move of Naxxramas (original) from Eastern Plaguelands to Dragonblight, Wartorn Cloth Scrap and boss tokens have been removed from the new (level 80) version of Naxxramas. Quest NPCs related to the aquisition of the item set remained in the Light's Hope Chapel to facilitate players to obtain the set.

With Cataclysm, all related quests have been removed thus making the set unobtainable.