Adoption records are filed in the county circuit court.
Adoption Registry, www.fccin.org/adoption-records.html
Adoption Record Laws, http://laws.adoption.com/statutes/indiana-laws.html
You might want to consider a DNA test. It doesn't provide an immediate answer but starts that process. There is a good article about it "Finding Your Birth Family with DNA", Family Tree Magazine, 12/17. Successful Searching!