The iTunes store has a featured song free for download every week. Additionally, you can rip CDs you already own using iTunes on your computer and load them onto your iPad. Apart from that I'm out of ideas. Perhaps other members know of free, legal sources.
I'll add here, for the record, that this forum does not condone piracy, so please don't suggest any of the more dubious locations. Enuksboy's question is valid, since we will assume that he is talking about legal sources.