If you're not required to use an SA certified helmet, then I'd look into an HJC. I love mine, and you can buy 'em at motorcycle shops so you can try it on for fit first. Cheap too.
You can get non-nomex headsocks for karting..
http://www.saferacer.com/spcoho.html
I'd check ebay for this you can probably score it really cheap there.