Typically you'll need to provide recent pics of your car/truck along with some initial paint scheme ideas you might have. There's quite a few people out there now doing really good renderings, my good friend just got one done by 918 designs, it was relatively cheap, good work, & short turn around time.
impulse
+1y
I went with Nick @ Surface Art for mine... did an awesome job.... might be a little more money, but he did more than I expected...
918 is putting out some awesome work these days too, and the price is unbeatable!!
mazdalow
+1y
918 has good prices and turn around (3-5 bussiness days) according to there website