Do you need:
Single Status Affidavit/Certificate
Certificate of No Record of Marriage
(Basically Proof of Single Status or Free to Marry Statement)
Go to the County Clerk's Office
50 W Washington St, East Concourse Level, 312-603-5656
Request a copy of a "Marriage Certificate." It costs $15 for the first copy and $4 for additional copies. They will do the look up and if you haven't been married then you will get a "No Record of Marriage Certificate."
Go to the Secretary of State, INDEX department, 17 N State St Set 1010, 60602, 312-814-8218. Bring the "No Record of Marriage Certificate" from the County Clerk.
They will have a form there for you to fill out. It is called the "Application for Authentication or Apostille Certifying Documents for foreign Use." Fill it out. The cost is $2 per document. You will then receive the Apostille, which contains a seal.
Hope this helps!