If you are unable to obtain copies of your W-2 forms you may fill out
Form IL-4852, Substitute for Unobtainable Form W-2, and attach it to your tax return. You must also attach proof of wages and withholding, such as copies of check stubs or a letter from your employer.
Your check stubs or letter should include the following information:
your Social Security number,
amount of Illinois Income Tax withheld, and
We will accept a copy of your
properly completed U.S. Form 4852, along with any required attachments, as proof of your Illinois Income Tax withheld from your wages.