There were two sets of Tier 5 for Priests, one for healing and one for DPS.
Tokens drop off raid bosses for Tier 5 sets, which were then turned in for the tier pieces.
Helm token – Lady Vashj in Serpentshrine Cavern
Glove token – Leotheras the Blind in Serpentshrine Cavern
Pants token – Fathom-Lord Karathress in Serpentshrine Cavern
Chest token – Kael’thas Sunstrider in The Eye
Shoulder token – Void Reaver in The Eye