The Palm phones use midi ringtones rather than the more popular mp3s.
The easiest way to get a midi ringtone on your Palm is to download it directly from your web browser to your phone.
The next easiest way to get a midi ringtone on your Palm is to email it to yourself, and save the midi. Or download it directly from the internet. I know that sounds painful, but it really is the easiest way to do so.
Midis must be 64k or less to be used as a ringtone on a Treo.
Sources for midi ringtone files: