Go to the lake, where all the portals to other areas are. Once you are there, you'll see Pandaren and their riding turtles. Near the Pandaren portal, there is a vendor for the mounts.
If you don't know where I'm talking about, go to the Dwarven district (not the similar area near the main gates of Stormwind). Behind the notice board (between the auction house and the bank) there is a small doorway that leads to some fields. Go out the door, turn right, go slightly up hill and look for Pandaren to your left as you reach the lake.