Yes, I know that the Rolling Stones concert at the Rockford Metro Centre happened almost 35 years ago (the anniversary is October 1st), but, it appears you can still obtain a ticket.
Tickets to an NFL game are, to be kind, fairly pricey. That's why once you've paid for them, you'll want to keep them in a safe place until you need to produce them for entry into the stadium.
The guy in the video below knows that...now...