Avatar Raiment and Avatar Regalia
Serpentshrine Cavern and Tempest Keep raids
Most bosses will require at least one other person
The set pieces are purchased using tokens which can be redemmed at Kelara in Shattrath City. The belt is made by tailors using a pattern that drops from trash in Serpentshrine Cavern but the belt itself is bind on equip so you just need to find a friendly tailor who has learnt the pattern.