Good/inexpensive site to download for MP3 portable

I just got my son an MP3 player for the holidays. (must be politically correct here)

I know about Napster, but that can be a little pricey for an 11 year old.

Are there good sites for him to get the music MP3 downloads that will work well on his MP3 portable?

I will be also very interested to find out how the sound is through my big system, but the real purpose is for his portable.

Thanks for any info.