What you need is a 3V Lithium battery #1632. While the dealer can take care of it for you - you'll pay a premium price for that service! Go to any place that sells a variety of batteries like watch batteries or hearing aid batteries (Radio Shack, etc.) and buy the battery.
Replacing it is simple. Open the case by inserting the physical key from the fob (press the button on the side marked "Push" and slide out the key) and twisting it in the slot. The battery goes into the circuit card on the underside. Just lift up on the card and you'll see the battery.