if you were an accomplished artist,
you'd already know what spraypainting a case design involved, So I'm guessing your not
possibly your best option is some kind of vinyl sticker design but that limits you to what others have already created as stickers
maybe a local design shop could do you something? promise to plug their services to try getting a discount
Well you could start with a model airbrush kit like you would find in stores for Testors models and paints. Or go to your local hobby store for one. You might even find someone good at it who would work with you or do it for you saving you years in deveolping your skills. Another idea would be to place an ad for an artist on a craigslist type site.