You can pretty much turn any song you have in your itunes library into a ringtone as long as it's not a DRM'ed .aac. What you have to do is go into "Get Info" for the song, and under "Options" tab, set the start time and stop time. I think there's a ~30s max. Then, create an aac version of the song.
These are not the full instructions -- but you'll easily find them when you Google.