IMHO, If you can find one locally, (or drive for a bit) that'll be the best way to go.
"Fitting" a helmet online is kind of tough, and you don't want to have to ship a couple helmets back and forth till you get it right.
Your head is a rather important part, and an ill fitting helmet is almost as dangerous as not wearing one at all.
After all that...there are
a couple good websites. (maybe you can fit locally, yet buy for cheaper online)
You might want to read my latest post in your regional forums:
That's entirely weird that your local club is being more strict than the SCCA in it's helmet requirements.