Most
clubs (like SCCA, NASA, etc) will have helmets to rent, but I don't know about the tracks. I wouldn't expect many helmets to rent, IF they are available.
You can buy $50 motorcycle helmets (M rated), but those don't usually fit well and they are heavy (comfort comes into play more on a 20 minute track session than a 1 minute auto-x run). A decent SA rated helmet will run you around $250 or so. I have a G Force, and I am very happy with it (it is probably the lowest that I would go). If you think you will only do 1 event or just 1 event per year, then you could probably get away with a cheap M helmet. But before buying a cheapy, I would ask around and see if you have anyone local who could let you try or borrow a helmet.
Helmets fit everyone differently, and if you get one that doesn't fit your head it sucks to wear. I stumbled on this link while Googling helmets:
Click. It is reasonably interesting. I believe SafeRacer sells helmets also. Attempt to try it one local before buying (if possible).
I borrowed my 1st helmet, and I would try that before spending tons of money if you don't plan on using it.
Josh