I think the part that 51Ford fan has shown is not the part your looking for. I think the part your looking for is really the shaft that everything is mounted to. I looked for this part about 5 years ago and found two by calling junk yards around the country. I do have a spare one hanging on the wall in my shop. It is a complete unit with fan, hub and shaft. I would have to look back and see what i paid for it.
I looked everywhere for the shaft and all i ever found was part 21A-8603 hub and pulley assembly.