Good luck finding one. The only thing I can think you could do is the junk yard on the west side (Arizona Mustang Mart). If they are still in business that is. Also, you can check National Parts Depot or Texas Mustang. That is a rare part and not very popular. I would suggest converting to a 8" or 9" rear end and be done with it. They have a huge parts list for them. Good luck,
Steve............