wht bolt pattern are the rims?
Brake parts should be available. easiest type of place to get brake parts is a relining/friction shop. they service the medium and heavy duty truck world and they can reline your shoes and arc them to fit your drums for cheap. They will have the books that show the Whl cyl part numbers. they sell the rebuilt frame mounted hydravac unit you mention. the hydravacs dont fail very often. Mine just did after 50 years. probalby just full of gunk inside but they are rebuilding it for 200.