There are many site which you can download music from, but they usually require registration or some kind of payment after like 10 downloads.
So...I found a way to get past that which is simple a easy and some of you should already know.
You must have Firefox for this, if you don't download it from mozilla-europe.org/en/products/firefox/
Anyways, once you have firefox all installed and loaded up, click on Tools > Options > Content > (Where it says "File Types") Manage >
Then once your at that stage, you'll see a list of all the different applications and what they run on, find MP3 and change it to save to disc or something like that.
After that, make sure everything downloads to My Documents and pay a visit to playlist.com
Search your song and find the url of it (Will end in .mp3). Once you have the URL, paste it in the address bar and search...it should start to download automaticly.
Enjoy,
Josh