Got mine here...
http://www.ebay.com/itm/05-06-07-08-09-MUSTANG-GT-BLACK-TEXTURED-REAR-DIFFUSER-/390153956733?pt=Motors_Car_Truck_Parts_Accessories&hash=item5ad6fced7d&vxp=mtr
Here's a better pic of it. It's a good fit and I installed mine about a year ago and no problems with it so far.