Virgin gives head...
Don't worry, you can take your helmet as hand luggage on Virgin (I did this last year) - I had it inside my empty tankbag which I used to carry all my other cabin/paperwork stuff - no problem.
As for buying a helmet out there - forget $40 cheapies (although I bought a Captain America open face for around $100 - kewl!) - treat yourself to a new Arai... they are not half price (dollars for pounds), but a significant saving - I was looking at a VXPro at $400 (£200), but had just brought my new Tour-X with me, so declined... will get one next time though!
Someone has already mentioned Cycle Gear - they are essentially the US equivalent of Hein Gerike here in the UK - top for choice and good value too...