Ive found in our area, there is a autocross every couple weeks somewhere nearby. There are about 5-6 major locations where they hold autocross events.
But like mentioned above, you usually have to join other car clubs or local SCCA. I wish there were more Viper only events,theres just not enough of us in one area. Check you local SCCA and you will probably find one event every month for starters, then if you find the "spot" they are held at, ig you go to that "Spots" webpage, you will find most other weekends there are events reserved with the different car groups.
http://www.scca.com/Inside/index.asp?reference=cendiv-west
Click on the appropriate regoin for your area. Then look on schedule for Solo events.
We are lucky enough to have a bay area website dedicated to Auto-Xing
Good luck and have fun.