You aren't missing anything, get 800 warzone commendations, turn 600 into mercenary commendations, buy a bag, get centurion marks and a chance at a random item. You can also do two dailies that give you a bag, and 2 weeklies that both give you 3 bags each. Once you reach rank 60, the only way to get battlemaster bags for the top tier gear is dailies/weeklies, and guess what? more random gear! how fun