That's quite a pump.

I'm guessing there's not too many oil fields in your neck of the woods, but oil field supply houses are also very good places to find stuff for pumps etc.
A lot of oil field equipment dates back quite a ways and there are a lot of different size pumps used by that industry.
There may be oil field pump supply sources on the Internet.

I'm sure you've thought about Grainger and McMaster-Carr.

Another place that may be able to help is a farm supply house.

Might check with a tractor store as well.
Lots of hydraulics on those and finding parts for pre-1950 tractors is easy.
A lot of these old tractors are still used on farms regardless of the farm size.