Open the website at http://www.findthecd.com.
Enter the name of the song into the "Track" field, then enter the name of the artist (if you know it) into the "Artist" field.
Click the "Search" button.
Search through the list of possible albums until you find the one for which you are looking.
Open the site at http://www.amazon.com.
Pull down the menu next to "Search" at the top of the Amazon site, then choose "Music."
Click inside the empty field to the right of the Search term you have chosen ("Music") and enter the name of the song for which you are searching.
Click "Go" to the right of the search field you have just entered.
Peruse the possible albums that might contain your song until you find a likely candidate, then click on the album artwork to see the tracks on the album. You may listen to tracks on the album to verify that it is the correct album with the version of the song you want.
Open iTunes. If you do not have the iTunes software, you can download it for free at http://www.apple.com/itunes/download.
Click on the iTunes Store link in the far left pane of the iTunes window.
Enter the name of the audio track into the "Search Store" field in the upper right corner of the window, then press the "Enter" button on your keyboard.
Peruse the possible albums from which the track might have come, then click on a likely suspect.
Listen to the song on the album you have chosen to make sure it is the same version of the same song.