Geoffrey Rosenberg at Alretta Truck Parts in Massachusetts (508) 788-9409 has tons of brake part knowledge. I bought all my brake parts from him including my remanufactured hydrovac from him a few years ago.

Did you try either of these?

https://harmonclassicbrakes.com/

https://www.brakematerialsandparts.com/booster-rebuilding - these guys relined all my old shoes