I make my own like dwfaust mentioned.
It's easy.
- Find any sound file you wish to use (or cut a portion of a song)
- In iTunes to convert the file to AAC
- In Finder, find the newly converted AAC file, right click and select "Get Info"
- Make sure you can see the file extensions (check the box to see them if you don't) and change the .m4a extension to .m4r. Hit accept when it asks if that what you want to do.
- Open the new .m4r file in iTunes and it will show up in the Tones section.
- Sync Tones to your phone and thats it. It will show up as a selectable sound when changing Sounds on the phone.