As users can create their own song for FoF there's just loads of them floating in every corner of the Internet, maybe some googling to find them?

I, personally use mostly FoF Wiki, Custom songs and 2828 Frets on Fire Songs, which make e.g. searching for songs of certain levels much easier. FoF Wiki requires you to be registered before you can download songs.

You can also import songs from Guitar Hero, if you happen to own one.

Installation instructions for custom songs can be found in the FoF Wiki.

Happy frettin'!