For a permanent MP3 you own, the Amazon Music Store or the iTunes Store are the most reliable.
Many artists release specific tracks for free download directly to fans. Download With Or Without You U2 MP3 – MuzicaHot
You can often find "name your price" tracks where you can download for free (or $0.00). Summary of Track Info for "With Or Without You" Artist Album The Joshua Tree (1987) Typical Bitrate 256–320 kbps (High Quality) File Size ~7–10 MB (MP3 format) For a permanent MP3 you own, the Amazon