1) Try on many helmets, like ^he said. HJC said I should be an XL. I settled on a M. Race shops are hard to find, so try out motorcycle shops.
2) Check and see if it has to be SA2010 or if M2010 is accepted, as well. M rating is generally less expensive. The only difference, I believe, is that SA rating includes fire retardation.
3) If you can use the m2010 rating check out motorcycle-superstore.com for good prices and great service.
Go: Cobb S2+, Big Tom FMIC, CP-E DP & DCI, Royal Muffler, AR OC
Turn: TC Kline/Swift SA, Vorshlag Plates, UUC Front Bar, H&R M3 Rear Bar, VAC endlinks & spacers, Forgeline 18x9.5/NT05 265 Square
Stop: Endless MX72, Stoptech Rotors/Lines, Motul RBF600