What if the song isn available on iTunes or Rhapsody?
If I can’t find the song (or an original version of the song) through either source, I put in a link to Amazon, where you can usually hear a snippet of the song and then consider ordering the full-length CD. I say “usually” because not all CDs on Amazon have listening samples — including all Beatles songs, for some weird legal reason. But I’ve done my best to find CD sources you can listen to.