Amazon has a reasonably priced one also, reviews there are excellent:
BMW E53 X5 E83 X3 Transfer Case Motor Gear 27 10 7 566 296GR Carbon Fiber Reinforced Nylon
I got mine from Odometer Gears:
www.odometergears.com
The fit was perfect and the part was great quality, though generally twice the price of some of the other options. I can't really compare to the cheaper options as I have not seen or used those (yet).