Each of these sellers will allow you to purchase a normal or epic version of a mount. The mount seller should be right next to the person you got your training with. Those late game quests also won't be available until the Phase 2 update. Time is money friend! These classes also have a quest chain that will give them the faster mount at level 60, but both of these quests cost a considerable amount of gold. I put free in quotation marks because they still require a time investment to obtain the mount. These two classes both can complete a quest chain that will give them a "free" mount.
You can get an additional 10% discount if you are Rank 3 in PvP, but this won't be live until a future update (Phase 2). You can get yourself a 10% discount on your purchase if you are Honored with the faction you are purchasing a mount and training from.
So, if you're an Undead, you'll only be able to purchase Undead Horsemanship and ride a Skeletal Horse until you upgrade your reputation with another faction. You can only purchase the riding skill that corresponds with your race, unless you are Exalted with one of the other Race's factions. You'll have to upgrade this skill in the future to ride the faster Epic mount. This should only cost you 20 Gold (16 Gold with discount) to get and enables your ability to ride a normal mount. The first and cheaper part of the process is purchasing your riding ability.