As long as your crash helmet is constructed from a fibreglass / carbon composite type material it will be okay to paint. You said your season is over so I'm guessing you use high end race crash helmets which will be fine for this.
The main thing is to use someone who you feel confident using, crash helmets are a safety device, it's therefore vital that the person doing the artwork and also the clear coating is aware of how critical is it to handle them properly.
Depending on your budget I would highly recommend Mike Fairholme who is a personal friend
http://www.fairholmedesigns.co.uk/
If your budget won't extend to his expertise I would also recommend Jo and Mark at Aerograffiti -
http://www.aerograffiti.co.uk/
Or Skin at Airyarts, I taught him everything he knows about crash helmet painting so you're in safe hands
http://www.airyarts.com/
There are plenty of professional crash helmet companies. All I would say though is talk to other people you race with, ask where they get theirs done. Looking at pictures on the net are fine but they also hide a multitude of sins. As I said about crash helmets are a safety device, if they need to do their job and protect your head in the unfortunate case that you do have an accident it needs to have been done properly and not by some muppet who's damaged the structural integrity of it that won't show from the outside, it will only show when it hits something with your head inside it, and then it's probably too late
