As mentioned above, these guys make' em:
http://us-mags.com
The wheels on the C-10 4x4 above: Scottsdale - U442,
http://us-mags.com/scottsdale-u442-w-6898.htm
Find a dealer here:
http://us-mags.com/dealers.cfm
They have a ton of cool wheels:
http://us-mags.com/wheels.cfm#restomod
This is my truck with the stock rally rims photoshopped to be larger. Not sure how big they'd be in this photo, but it gives an idea.