Trying to get a copy of a Marriage Certificate.

Hello, I’m Jason, and my question today is how do I get a copy of a Marriage Certificate. Not for me, but, for my niece who is 18 years old. My niece is 18 years old and she is trying to get a copy of her mother’s Marriage Certificate from when her mom got married and when she got Divorced. The date from when her mom got married until when her mom got Divorced. The city where her mom got married was, I believe was Waukegan Illinois and the city where her mom got a Divorce was Waukegan Illinois. My niece would ask her mom, but her mom has been in and out of prison and she doesn’t have it any more. So what should my niece do? Or what website does she go to?