If you cannot manually locate the song on Google Play, you can download the song from iTunes or Amazon after you've purchased the song and then use the Google Play service to listen to them. Google actually is running a promo on this right now.
The site says, to use iTunes on Google Play do the following:
Google Play helps you bring your iTunes library to your Android
devices. You can upload up to 50,000 of your songs from your computer
to Google Play for free. Once you’ve uploaded your music, it's
instantly available on the web and your Android phone or tablet. No
wires, downloading or syncing.
Sign up for your Google Play music library To get started, sign up for Google Play and download Music Manager, a simple desktop
application that lets you upload your collection from your iTunes
music library or any music folder on your computer to the cloud.
Sign in to Music Manager, choose where you keep your music and it starts uploading automatically. And when you add new music to your
computer, Music Manager can automatically add it to your library too.
You can upload 50,000 of your songs for free. Music Manager even adds
your iTunes playlists and ratings so you can start listening to your
favorites right away.
Now that you've set up your Google Play music library, you can access it on your Android phone or tablet and the web. Sign in and all
your music is just there, like magic. And when you upgrade to the
latest Android device, your music comes along too. You'll never be
without your favorite artists again.