An actual chroming shop would probably charge you an arm and a leg to do a helmet, because they add-in extra $ for a cushion in case they destroy your part. Basically, they paint on a thin substrate that has metalic powder in it, and then put your part in a copper, and then chrome bath with an electrical lead attached so that the chrome bonds.
There is laquer model paint that is a tad bit better than what you can get in a rattle can. I have not used it yet, but had a friend that has used it on an a remote control car model.
Look under liquid metal at spazstix.com
The other is at alclad2.com and I don't know of anyone that has used it yet.