The Black War Mammoth mount is sold by Stone Guard Mukar (Horde) / Knight Dameron (Alliance). Although to purchase the mount you’ll need two things, the first is 15 Marks of Honor, the second is for your faction to be in control of the PvP zone Wintergraps. If your faction doesn’t control it then you won’t be able to purchase the mount until your faction does.
The mount is essentially the same for Horde and Alliance, the only difference being the armor trim colour is different depending on your faction.
The best way to gain 15 Marks of Honor are the PvP Brawls but unfortunately they are an event so aren’t always active, check your ingame calendar to see when the next one will be. Next best way is Rated PvP (Arena and Rated Battlegrounds), if doing rating PvP isn’t your thing then doing Epic Battle Grounds are a good source. Although if you’re in long queues, long bgs and also losing, then doing normal battlegrounds will be the final best source.
Once you have your 15 Marks of Honor then head over to your Wintergrasp in Northrend. If your faction is currently in control of Wintergrasp then head over to the large structure in the north part of Wintergrasp. To the right of the stairs you will find your factions vendor.
Simply speak to the vendor and purchase your Black War Mammoth for the 15 Marks of Honor.