I thought I read a couple of years ago that you can send your old one to The Mustang Corral in Edwardsville, Illinois and they can rebuild yours for you.
Also check
Mustang II Network - 1974 - 1978 Ford Mustang II - classifieds section. You might find one there as well, or find someone who can help located one for you.
Finally, check out Phil Schmidt's Mustang II Specialty Shop (there is a link to him on
Mustang II Network - 1974 - 1978 Ford Mustang II.) If Phil has one, it might be expensive, but since they do not reproduce these (DA Ford didn't release the patent on it), it may be your only option.