Without hacking, the legit way will cost $1.98: Find a song with a phone ringing in it, buy it in iTunes, isolate the ring, and buy the ringtone. (You might be able to rename the ringtone so it doesn't get listed as the song title; I haven't tried that.)
Anybody know of a good candidate song? I know +Sometimes a Fantasy+ by Billy Joel has an isolated ring in the beginning, but it's the ringback tone you hear when calling someone else.