So you want to buy your own helmet? Pretty much the only helmet requirement for AutoX (every one that I have attended) is that it must be Snell rated within the last three revisions. Snell ratings are renewed every 5 years.
The following Snell ratings are good:
SA95, M95, K98, SA2000, M2000, SA/K2005, or M2005
Many motorcycle helmets aren't Snell rated, so make sure you're getting the right one.
I buy my helmets from www.helmetcity.com. I personally have the HJC CS-5 which is an open-face. Right now they are doing free two day shipping so you most likely can get a helmet shipped to you for around $70.