A HUGE can of worms but here goes....
First, ask the helmet manufacturer or local re-seller for a painter reference. That will help pre-qualify the participants if there are still any left out there. The mfr. or dealer can advise if top/clear coats are acceptable and if they will compromise the outer shell or not.
This is a big issue, do your homework. If you look at the Troy Lee designs, they are mostly water transfers with a clear over top. There is a reason for that. I would also check with the local race authorities and their technical inspectors/scrutineers for comments or suggestions about helmet care, etc.
Second, if you still want to proceed find someone who uses water-borne paints only! The new PPG enviro series or similar. Airbrush shops sell water borne paint, they may have a local guy who still paints helmets. Reason - the fumes from solvent based paints will eat the foam inside your new helmet, no matter how well you mask it off. This is an obvious safety issue.
Third, if you still want to proceed make sure that whoever sands or preps your new helmet doesn't scratch or gouge the surface before they paint it. I wouldn't use anything rougher than a green scotchpad and then only with the very lightest pressure. And don't forget the snaps and frontal head-restraint hardware. If you remove them put them in a zip-loc baggie so you don't lose them when time to re-install.
Fourth, consider vinyl decals. Not to wrap the entire helmet but stripes, lines, scallops, numbers and flames are possible now with digital printers and contour die-cutting. Avery or 3M are best - their vinyls are easy to install, easily removable with a heat gun and colour-fast.
Good luck with your search.
Lorne (Sign Guy)