I bought one from Brothers Trucks. It fit great except that I had the wood side steaks made and now it doesn't work. So, it is like brand new and I have no use for it. The other problem is I have a long bed. It is an out door heavy duty one. I don't see it listed on their web site anymore.
I also bought one from
www.carcoverworld.com for my 2004 Expedition. It is their Weather Guard cover and is of very good quality. However, it was $550 bucks. They still came in cheaper than California Car Cover and they shipped it in a few days. It fits great and I am very happy with it.
Because I have the side stakes on the '59, I ended up having a local upholstery shop make me a custom truck cover out of the same material as the one I got for the Expedition. They did a good job and it cost about the same. So, you might try that route. Look for an upholstery shop that caters to boat owners. They have more experience with the covers.
Good luck with finding a cover for your ’56!