Might be outdated due to the new release in pvp with patch update 2.0 and arenas.
Battlemaster Commendations have a random chance to drop from the Battlemaster Gear Bags. The bags are obtained from completing PVP daily quests and having more than 50 valor. The Battlemaster Commendations can also be bought for 1,000 Warzone and 1,000 Mercenary Commendations from the PVP items vendor.
See also: PVP Gear for an explanation on the PVP currency system.