I bought my G-Force full face Snell 2000A (Automotive) from here:
http://www.fullthrottleraceparts.com/index.htm
Prices was the best I could find on the net (~$250). Shop around, the only drawback from buying on the internet is you obviously can't try them on. Although you need only to measure the circumference of your head just above your ears to get the size. Mine fits good and am pleased with the quality. They also carry the Simpson line as well.
Ron