If you are working with a Realtor, she can access the information for you if the sale was an MLS sale.
Otherwise, if the sale occured at least a month ago, you can go to the Cook County Recorder of Deeds website and see the Deed info, which includes the purchase price and grantor/grantees, online: http://www.ccrd.info/CCRD/controller
If you don't have the PIN number, go here first to search for the PIN by address: http://www.cookcountyassessor.com/filings/searchflat//search.asp
The link below should help you as well.
Customer Service Representative