Legionnaire's Plate Leggings

The Legionnaire's Plate Leggings are the plate legguards of the Horde PvP-Rare Warrior set called the Champion's Battlearmor.

Source
This item can be purchased from First Sergeant Hola'mahi located in the Hall of Legends in Orgrimmar. It costs 6375 Honor Points and 30 Warsong Gulch Marks of Honor.