The Slayer's items are sold by Tydormu in the Battle of Mount Hyjal instance. They are "purchased" with Tokens of the Forgotten Champion.

Slot  Token  Dropped by  Found in 
Head  1  Archimonde  Battle of Mount Hyjal 
Hands  1  Azgalor  Battle of Mount Hyjal 
Legs  1  Illidari Council  Black Temple 
Shoulders  1  Mother Shahraz  Black Temple 
Chest  1  Illidan Stormrage  Black Temple