How Can I get child support that is owed from my deceased ex?

You must file a claim against his Estate while it is in Probate. It is NOT the job of Child Support Enforcement at this point. You must get your own attorney to do this. You can get your own copy of his death certificate by contacting the Circuit Court where he died. They may direct you to the Vital Statistics Office, or they may have it on file. It's easy to do. Your attorney can get it.