Furious Avatar Legplates are an epic-quality equipment item that are only available to Heroes and are sold by Requisitions-T2H. Once equipped, they unlock the Avatar Bombardier legs costume style.
Source
The Furious Avatar Legplates are sold by Requisitions-T2H, a vendor found within the Hall of Heroes between the Meta and Magic Wings of the JLA Watchtower. The item costs $472 and 85x Mark of Conquest.
Vendor item
Style unlock
Upon equipping the Furious Avatar Legplates, a character unlocks the Avatar Bombardier legs costume style, which can be accessed at any time from the Style Window.