It looks as though they have been discontinued
However, I did some research, the tires were made for Sears by Cooper tire. Interestingly enough, it looks like Cooper has 3 different tire series that seem to share the same casing (basic tire carcass) as the Viper. Cooper has the Cobra GTZ, XST, and GTH. All 3 of these tires have a 9.35" tread width, the GTH seems to be the same (although tread pattern is different) as the Sears Viper HX, same UTQG rating 400/A/A too. According to the info on Cooper's web page the XST should provide better traction based on its UTQG rating. Hmmm? Viper.....Cobra? makes sense 
Check out:
http://www.coopertires.com/tire_cooper/passenger.html
Thx Brad
Check out:
http://www.coopertires.com/tire_cooper/passenger.html
Thx Brad