Everything you need to know is at:
http://www.hansdevice.com/choosing.html
I got the MODEL 20RE
20: for an UPRIGHT SEAT (NASCAR, Busch, Sprint, Midget, Quarter Midget, Drag, Modified, Bandalero, Sportscars including Porsche, BMW, Corvette, Honda, Viper etc.)
R: 15 to 18 Inches neck - Size Medium (M) (Called size REGULAR on some models)
E: Economy version. It is available in lightweight (Professional) and regular (Economy) versions. The only difference is weight - there is no performance difference between the versions. The weight difference is only four ounces and considering the harnesses pull down with much greater force than four ounces, this was a no brainer because the economy version is $270 cheaper.
Leigh bought it through Mark Hicks (another NSX owner) who is a distributor. I believe all HANS distributors sell at retail. The 20RE cost $865.
Bob