Here are some sources for music that you can use without any copyright infringement:

 Magnatune
If your podcast is non-commecial (or any school use), you can download 128k versions of the songs for free. Click the "license" button next to any button and click the "Non-commercial" option. Save to disk. podsafe music network
Free account." You must be logged in using your podcaster account to download music. The process is achieved in two steps; adding a song to your playlist, then downloading the song from your playlist."

podsafe music network
Free account." You must be logged in using your podcaster account to download music. The process is achieved in two steps; adding a song to your playlist, then downloading the song from your playlist."