Unfortunately anything over 9.5 wide in 18's start to get expensive , as they have to cut two hoops and weld together to make a 12" wide wheel. Your not going to find an "off the shelf" 18 x 12 to fit a C3 (I'm guessing you have flares, so an 18 x 12 with a 5" backspace) so they are goign to have to be a 2 or three piece wheel, made to order.
Any of the billet wheel companies (CCW, iForged, Boze, Bonspeed etc) will make them for you, but your talking $800+ a wheel.
Nick