[JMay]

dvd3500,

Would suggest you have a Net trawl using the extension STL file i.e.

Porsche 924 STL file

this yields several options ranging from a few pounds to £50 plus for a fully rendered file.

Otherwise you can try scale model & 3dp forums where you could put requests onto, as there are many gifted designers that can draw these shells from scratch, derive them from computer games or scan from models and there maybe files out there not published that you could use or someone could adapt for you. Bearing in mind this option may be the most costly or maybe if you are lucky enough find someone who is interested in these shells and will do it for their own use.

Cheers
             John