There's always a set of late '70s Lincoln rear disks for a 9". They are a little harder to find, but they are all Ford and bolt right on. I am not sure what you need to do as far as rotors go, but I am sure that someone on the board has a set and has all of the details.
You can also use a set of rear disks off of an Explorer. The Ford Racing conversion kits for the 9" are based on these. You can look them up on the Ford Racing website.
I run the Cadillac rears. They are a little fussy to set up, but they work great and are dirt cheap to get.