Get a program called YouTube Downloader and copy the URL bar into the program and it will save automatically. Or you can check to see if the song is on ITunes.
if you want you can go to 4shared.com and type any songs or videos and then click on it and download it. 4shared.com for me is the best website since limewire doesnt work anymore. :D piece out.
You can not legally buy a download on the Internet, but you can buy a Compact Disc (CD) from Amazon or many other sites. If you need it in MP3, WAV, or FLAC format you can use a CD ripper such as...