I went to Roadrunner records website and found about 200 songs available to download for free legally.
Granted, most of these new metal songs will not appeal to most, and I do not care for songs that sound like they are being sung by the Cookie Monster, they were free.

If you are looking for specific songs, you might be able to legally download from Itunes, or the record companies website.

iTunes

cheap bastard

If like me you listen to music that is not usually available via legal download services like iTunes, the best bet is to download the full album, if you like it, buy the physical disc and let it gather dust after ripping to a high quality audio format... thats my standard line anyhow.

used record stores are good

i think its a really lame idea to pirate someone's original art.

but then im going to assume you're asking about legal download services.

itunes is wonderful.