Of course, to be serious in your recreating of Hoth, you need more than one. Wizards of the Coast is producing a limited release of AT-ATs for its Star Wars Miniatures game:
The model stands 14" tall and is pretty much completely inappropriate to the skirmish scale of the game. Looks nifty, though. It might work better with a modified rule set designed for larger battles.
Were I to buy one, I'd go to my favorite SW minis seller, Auggie's Games n Movies
Neat model. I have no idea what I'd do with one. Now, if they had a vehicle-scale game with 1" long snowspeeders, I'd be right there.