Laser Horizons sells laser cut styrene "car sides". Said sides are designed to be mounted to pre-existing passenger car bodies (either American Ltd passenger car "core" kits, or mass-manufactured cars from the likes of Kato, Con-Cor and Bachmann). LH's catalog is extensive, covering numerous different roads and car styles. As for where/how to get them? Well, the late Jerry LaBoda used to provide a nice car list (along with ordering instructions) on his N Scale Varnish website, but unfortunately that's all gone now.
Got a picture of a finished car? Do please send it to me!