Go to the dealer and ask for it. They have it in big drums in the back, so you'll probably get a few puzzled looks and the rep will try to convince you that:
1. You don't need to replace it.
2. They don't sell it.
but if you are persistent enough, they'll sell it to you (you need to bring your own container) for about $7 - $8 bucks per quart. Just make sure you know the part number - there should be a sticker on the pan saying what fluid to use.