Can be bought with Warzone Commendations from the PVP Items vendor; 30 Warzone Commendations will buy 10 Mercenary Commendations. You can also find Mercenary Commendations in chests in the Outlaw's Den on Tatooine.
Champion Gear Bags can be bought from the PVP Items vendor for 200 Warzone and 200 Mercenary commendations. The bags have a chance to contain an "Unassembled Champion ...." - a token to trade in for a PVP item.
See also: PVP Gear for an explantion of the PVP currency system