As soon as you know your size I think that SafeRacer has a good selection of helmets and if you google a bit it shouldn't be too hard to find a 10% discount coupon online
Otherwise, I think the guys at Wine County Motorsports (inside Infineon Racetrack, so don't go there this weekend as there will be NASCAR madness) are top notch and actually have good prices
Actually: One more thought:
If you can wait with buying a helmet then you should: By the end of the year the new Snell SA 2010 helmets will come out and I'd imagine that the older standards will eventually no longer admitted by PCA.
I am pretty sure that next season SA 2010 will be a requirement at least for DE/TT not sure about A/X though