From my past experience of shopping for one, ebay if your best bet. Or a junk yard, but that won't have install hardware. Hit up some on car-parts.com. I have a scan of factory install instructions if you find one on junk yard.
what color? i think they are pretty cheap from the dealership, or at least they used to be if you had an oddball color like taupe. got mine real cheap years ago.
FYI, there's 2 different styles. one that sticks straight out from the roof line and is usually body color, the other that just pulls the air down behind the hatch to keep it from swirling and is almost always black.
Im looking for one that comes back off the roof line, there are some minor dents from previous owners luggage i would like to cover. Color is 466 - Deep Sapphire Prl