What state do I file a petition to legitimize when I can't find the mother?

My son was born out of wedlock in 2002. I have had liberal visitation with him until recently (year or so). I feel that it's best to involve the courts. My problem now is that I don't know where she lives. Her last known address was in Georgia, however she's since moved from the address that I have for her. When dialing the last known phone number for her there isn't answer. I am aware that I need to have my son legitimized and am waiting on the hardcopy of the paternity test that was taken in 2003. That proves I am indeed his biological father, but where do I file? I spoke with an attorney in Georgia who says I cannot file in Georgia because it can't be proved that she still lives in the state. I also spoke with an attorney in Illinois (where I live) who says I can't file here because she needs to be served. I can't afford an investigator and I've exhausted all internet searches as it only reveals information that I already have.