I recently upgraded to Ringo+ for using MP3 ringtones.
Somehow I remembered seeing a thread that said a great source for MP3 clips was Walmart's online CD store. I checked it out and found that they have a huge, searchable selection with 30 second "samples" of the tracks that you can download ( right click and save), then edit using Audacity (MP3 editing freeware). In particular, there's a ton of great ringtones candidates in "sound effects" CDs and some great stuff in "Monty Python" CDs.
...My wife's ringtone is Brunhilde singing in the Ride of the Valkerie by Wagner (the helicopter music in "Apocalypse Now").