Here's a nice page that was sent to me by another researcher:
http://www.itd.nps.gov/cwss/
14 listed with the Cassel surname
23 with the Cassell spelling
2 with the Cassil spelling
3 with the Kassel spelling
I believe that at this time - this database is only African-Americans (but I
could be wrong)
Another site worth surfing is the IRAD site for those searching their
Illinois roots.
http://www.sos.state.il.us/depts/archives/database.html
Pat