I would recommend Hellion Design in Westfield, MA. Lee Helliwell does some phenomenal work. He painted my lid for me. If I get the chance to take some pics of it, I'll post them. I don't have any right here to post now though. The number for Hellion is 413.568.1638. Give him a shot. He paints a lot of helmets for motocrossers in New England. He's done everything from just helmets to complete bikes and I believe cars.
Depends on what you're looking to get done for paint. My helmet cost me about $400-450 for paint alone. They usually recommend starting with a white helmet that's new for best results. Painting helmets takes quite a bit of patience and experience since you're painting on a constantly curved surface. Also knowledge of materials is critical since the wrong materials can damage a helmet and render it unsafe.