Perhaps the dynamics of the situation, reality-based, is that at this point in time, finding a good used booster that still works, that's not on a car or on a car that "used to run", would be very slim. It's not 1969 any more, unfortunately. Wish it was!
I suspect the only option might be to find a salvage yard item and get it rebuilt. Even if the salvage yard item might hold vacuum, it could fail reasonably soon due to a brittle diaphragm. They aren't that bad to change, but having to do it twice is not desired.
Sorry, just my reality-based comments,
CBODY67