I can't help much with the original seal materials but I do know a lot more about modern seal materials. From most of the info I've seen about the Hoo-Dye shocks, they have a large round seal to fit the screw on reservoir cap to body joint but I don't know if it is a round cross section like and O-ring or not. If it is an o-ring then a person may be able size it to existing aerospace type o-rings from Parker Hannafin. The smaller seal between the outer cover and the inner cover appears to be flat cross section with an inside and outside diameter to fit the angled recess there. They could likely be fabricated from rubber sheet if the person knows the thickness and what size to cut them to. The little one on the valve is a mystery to me since it isn't well illustrated on the diagrams I've seen. The shocks were throw away items back in that era so they were never planning on ever selling a rebuild kit.
The Gold Coast Model A Restorers Club has some interesting ideas about salvaging some not so badly rusted units but it sounds like it would not only be a good bit of work but also some triumph and some tragedy in trying this method. It's interesting though.
http://www.goldcoastmodela.com/Shocks/hreb.html