The Snowshoe Rabbit vendor is at Amberstill Ranch in Dun Morogh, north of the road between Kharanos and Ironforge.
This is what the ranch looks like from the road:
This is the ranch, around back of the house in the above image. It's where you buy ram mounts, and the person you want is that kid chasing the rabbit (you can see them in the foreground):
The most frustrating part is CATCHING her! She's constantly moving and you have to chase after her like a mad mini-game, trying to right-click! Once you do, she'll stay still for a little while, and you can buy the rabbit at your leisure. 20 silver apiece. I got two, one for Birdfall and one to stock away for my Draenai mage, who seems like a rabbit-lover.
Very cute, fat bunny. I like him.