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. And no, I'm not going to try to cover them all here. If interested, Jerry LaBoda has provided a nice list (along with ordering instructions) on his website - N Scale Varnish
Got a picture of a finished car? Do please send it to me!