Hmm, interesting request. For years it was the other way around.
But to answer your question, you can use any sond bit as a ringtone. (as long as they are in one of the supported formats: AAC, AAC+, eAAC, eAAC+, MP3, OGG and WAV).
MP3, WAV and OGG are the most common and easiest to find. I did a quick google search for "Old phone ringtones" and some old midi tones popped up. (You have to convert those). Good luck