How do I export one song from iTunes to a .mp3 file?

You can use iTunes to convert any copy protection free songs in your library another format. When you convert a song in iTunes it will be converted to the format that you have set to import your CDs in. Converting creates a second copy of the song in the format you have chosen which gives you the opportunity to either keep or delete the original. If you are using iTunes 8 go to iTunes>Preferences>General and click on Import Settings,, Change your Import format to MP3 Encoder for example. Go back to your library and highlight the songs you want to convert and go to the Advanced menu at the top of your iTunes window and choose "Create MP3 version": iTunes: How to convert a song to a different file format

Click on the MP3 version you've created in iTunes to highlight it and drag it to your desktop to create a copy if you want to use it in another program or put it in another folder.

If it's an iTunes Plus song or came from a place other than the iTunes Store, open the General section of the iTunes preferences, click the Import Settings button, change the default encoder to MP3 if it isn't already set to MP3, control-click the song, convert it, control-click the converted song, show it in the Finder, and retrieve the MP3.

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Try this AppleScript (courtesy of Doug's AppleScripts for iTunes):

http://dougscripts.com/itunes/scripts/ss.php?sp=convertandexport

It's a bit old but it may still work.