Carwalls is a site I frequented a year or two back. The place is GREAT if you fancy yourself a bit of an auto guy/gal, as they sort wallpapers by make and model - and especially useful, they provide them in over a dozen different resolutions, all the way up to 2048x1536!
Here a couple I have saved from way back, and definitely some of my favorites:
Until you can own these cars, this is the closest we can get! :(
Hope you guys manage to find something you like!