The way I understand it, you are asking two different things. Index racing is different than class racing. Most of the time they don't mix.

You have a strong group of class racers in the IA, MN, and IL area. They usually will post their schedule on this site. The schedule isn't done until the first of the year after they find out what NHRA and IHRA are doing.
http://classracer.com/classforum/forumdisplay.php?f=3

You also have this group that does both but I don't know a lot about it.
http://www.nssracing.com/


Now index racing just runs off an index. The most popular are 6.50 and 7.00 in the 1/8. It is real popular in the southeast. Not sure about the Wisconsin area. Some let tube frame cars run, some are limited to back half rules. The rules that are almost universal are no electronics or timers. You can run a throttle stop but it must be static. Seems like 6.5 and down down run off 4. pro lights and 7.0 up use a .5 starting light.

You can find out what is going on by watching this site. They don't have the Internet police so it is a little crude and not NWS. Some blockers won't let you in. There is a group trying to get some races for the midwest area but have not gotten on the thread to see what they are doing.
http://www.yellowbullet.com/forum/forumdisplay.php?f=11

Hope that helps.