Depends on what you need.
The 1965 to early 1967 used a 1" sector shaft, the later 1967 to 1970 used a 1 1/8" sector shaft. New ones are hard to come by. There is a place that used to Ebay the 1" sector shaft, but they were Argentine repros made many years ago and they were about .003" oversize and wouldn't fit into the needle bearings.
I have seen Mustangs Unlimited sell NOS Ford Long Input/Worm Gear and Rack assemblys for the 1965-1967 Mustang with 19:1 manal steering, so I believe they must have a limited stock of them.
Have not seen anybody with a regular stock of short input setups, long 16:1 setups or sector shafts.
If you want new, you will have to go with the Argentine built Flaming River box.