Very complicated and confusing area, so I will try not to go overboard. There are various helmet standards, they lowest being DOT certification and the highest being Snell Foundation. Snell has multiple standards...so for autocross/track days look for a helmet with a Snell SA2010 rating (The 2015 standards have been released but I do not know if anyone has a helmet out meeting them). SA stands for "special activities", Snell speak for auto racing. A Snell M2010 helmet is for motorcycles and they have different standards. Although buying on the internet may be cheaper, as Nik17 points out, FIT is all important! Talk to your local SCCA region to find out a local store. In Milwaukee, my area, Pegasus Racing has a great selection and top notch staff.