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Home - Harmon Classic Brakes
They list all three boosters, as well as parts and rebuilding service.
I don't think they're listed, but I would contact them about the reservoir and correct check valve- who knows, they may have something sitting on a back shelf. Or try a "Parts Wanted" ad on this site; it's free.
Honestly, by the time you source the rest of the parts and get it put together, you could find out the booster needs rebuilding... why not just get and install Dr. Diff's booster and brackets? All brand-new, and no reservoir required.