Can anyone refer me where I can read up on IL law making it mandatory to provide customers with a written service provider list with my gfe?

It's not IL law, it is a RESPA requirement. It's buried in the instructions for completing a GFE:
Appendix C To Part 3500 —Instructions for Completing Good Faith Estimate (GFE) Form
Where a loan originator permits a borrower to shop for third party settlement services, the loan originator must provide the borrower with a written list of settlement services providers at the time of the GFE, on a separate sheet of paper.