Napster is by far the best way to search and retrieve MP3's off the web. It works like an online commutity with you loggin on. When you log on you have acces to thousands of other member's MP3 collections and they can download from you. Sharing is such a wonderful thing!
Audiofind is another great place to search for MP3's as well as other media types. It is very easy to use an provides many good hits on popular titles. Give it a try!
Scour.net is a broader search engine. Instead of just looking for MP3's, Scour searches for all types of online media. While searching under music you might find a lot of .wav files and other formats. Downloading the Scour Agent can also be an added help when using this site. It helps download speeds when downloading for the site.