You can download the song "I Still Haven't Found What I'm Looking For" by U2 from several websites that offer free mp3 downloads, including:

• 101.ru, where you can listen and download tracks from the album The Joshua Tree.
• Muzoto.com, which provides a free mp3 download at 320 kb/s quality.
• Music-hit.com, also offering free mp3 downloads and online listening.
• Mp3party.net, for free mp3 downloads and online streaming.
• Audio.com, with a version by BIRDNERD available for free streaming and download.
• Various other sources with cover versions or remixes, including versions with Scarlett Johansson or Tommee Profitt.

If you're looking specifically for the U2 original, 101.ru and Muzoto.com are reliable options to get the mp3 file for free.

Please note to use legal and safe sources for downloads.