As of last November, the state of Illinois has changed their laws regarding adoptions. Adult adopted persons (21+) born in Illinois can request non-certified copies of their original birth certificates through the Illinois Department of Public Health.
There are slight differences in the laws depending on whether the adopted person was born before 1 Jan 1946 (birth parents of children born after this date may request that their names be deleted from the non-certified birth certificate within their lifetimes, and all birth parents may indicate their preferences regarding contact with the adult birth children) but this new law is still the most open that the state of Illinois has ever been.
Go here to read more about the new laws:
6 minutes ago