Believe it or not, some legitimate channels like Rajshri Tamil or Ap International have acquired rights to old films. Search for "Sathi Leelavathi Full Movie" on YouTube and look for verified checkmarks. These are often ad-supported but free and legal.
Sathi Leelavathi is a masterpiece of Tamil cinema that deserves to be watched in good quality to fully appreciate the nuances of Kamal Haasan's performance and K. Balachander's direction. While search terms like "Isaimini" are common, opting for legal streaming methods is the safer, ethical, and higher-quality choice.