Amazon Music: A link is provided on the song page under "Purchasing options".  If the link fails, try Search for Mediatrix Media which should display the songs available from Mediatrix Media.  Find the desired song and click on the "MP3 Music" tag on the right.  If that fails as well, browse to amazon music and search from there...

iTunes: Those who already use iTunes can use the usual method for finding music there.  For Windows users, there is a free application called iTunes for Windows available at the Microsoft Store that provides access as well.