Mark:
I picked up a helmet from Wine Country Motor Sports in Jupiter FL (
www.winecountrymotorsports.com) last week. I am a complete novice, so I went to the store to be fitted. (Going to the Reptile Roundup this year).
I went to get a Pyrotech helmet as it was recommended and the price was good for a first helmet. I could not fine one to fit me correctly. Depending on the helmet brand I was a medium or a small. I ended up with a Bell that fit very well. Price was $400+/-. These guys specialize in Race Gear and Equipment.
My advice is that you should try to go somewhere to be fitted. Trying someone elses helmet for size may not work. It's too important, not to have a correct fit (comfort, visibility).
Just my two cents. I'm not by any means an authority. Perhaps more experienced drivers will have a different opinion.
Good Luck. Since your in Florida, I'm sure we'll meet up sometime this year. Just purchased my trailer for the trips.