If your former supervisor has retired or moved to a different company, give her a call or reach out privately on social media to see if she might still be willing to act as a reference for you. You can still use that person as a reference.
Sometimes you may have no idea where your former supervisor has gone. If you worked at a larger company with a human resources section, it will keep files about past employees. A potential employer can always contact them to ask for details about your time at the company.