TireRack (doesnt have one for our cars), Auto Addictions has a GREAT one for tbirds (not cougars though), Pepboys but its not that great, and rimulator (again, only birds. But wider selection of wheels then Auto Addictions). Theres an phone app called Wheels On that does the same thing but it takes a photo of your actual car.
Most of those sites suck, I've found even a quickie scaling and cut and paste job using MS paint will give you a much better idea of how something will look on your car. I can get a better picture in my imagination of how the car would actually look than on those generic wheel rendering sites.
And IMO certain wheels just look better on Tbird's than Cougars and vice versa.