I bought mine online at Pacific BMW. Part No. is 65-12-0-403-658.
http://pacificbmw.com/
There is also a company selling them on eBay - just make sure you get the right part number.
Keep in mind that after you do the hardware install you will still need to take the car to a dealership and have them activate and program the alarm. My dealer (where I bought the car) charged me $100 for activation.