I hate how full of **** dealerships are sometimes.
You can order the unit through any dealership parts department. How else would somebody replace one if it broke?
Here is a listing of the OEM components/part numbers in question:
280 Audio & Visual :: Body Electrical :: Genuine Nissan Parts :: 370Z Parts (Z34) 2009-2010 :: Nissan Parts, NISMO and Nissan Accessories - Courtesyparts.com
Honestly, it would be much more expensive than buying a better aftermarket unit and having a good car audio shop install it.