No longer available to players. If you want the look, try Gurubashi Punisher.
Mallet of Zul'Farrak was a rather involved quest chain that took the player into dangerous places (if the player was doing the quest at-level). The purpose of the item - it wasn't weapon prior to Wrath of the Lich King - was to enable players to ring the gong that summons Gahz'rilla. The mallet became an equippable mace in Patch 3.0.8 when it was no longer required. It was not removed from the game until its associated quests were deprecated post-Shattering.
Dungeon groups forming for Zul'Farrak especially loved those players who had the mallet, so it was a worthwhile endeavor to acquire it.