Advantageous is a script that lets you search the iTunes store and then buy the MP3s on Amazon.
Browse through the iTunes Store, just as usual. Or use iTunes Genius feature to find new music.
When you find something you want, click on Get MP3 from Amazon in the iTunes Script Menu.
Advantageous mp3 takes you to the artist's or album's Amazon MP3 site. You can buy the music DRM-free and in better quality.