I would expect you can get them from anywhere although to tell the truth, I've never purchased a ringtone. Much easier to take a 30 second clip from any music on your computer and upload that clip to the phone and use it as a ring tone. Any short MP3 will work. (Well, you could technically use a longer one but the phone doesn't ring more than a few seconds and with a shorter clip, it's easier to get exactly the piece of it you want for the ringtone.) I use Roxio Creator (full version, not what comes with BB DTM) to edit the MP3s.
Personal: iPhone on AT&T
Work: BB 9650 on Sprint