I think you might be asking alot for such an old vehicle. Doubtful anyone carries a hard cover for our trucks any more and certainly not worth production being that very few would be sold. You might luck out and find one on CL, but I have a feeling the condition wouldn't be good.
I think bucket made the best suggestion. Make your own out of plywood and take it to an upholstery shop. I'm sure they can stitch a nice vinyl cover over the plywood.
As far as that goes, if you know how to do fiberglass work, you can make a hard shell out of it and have it painted any color. High dollar though considering it's purpose.