You can go to the Cook County Recorder of Deeds, if the property is in Cook County. You can look up the info easiest by a pin # search.
To get the Pin #, you can go to the Cook County Assessor's Website. Go to the Online Tools tab and select residential property search. Enter the address and city.
Information on these sites if fairly up to date and rapidly updated in my experience.
If the Buyer is a LLC or Corp., you can go to the IL Secretary of State's website and do an LLC search to get contact info.
Best of Luck.