Does anyone know where I can purchase an HSMO green reservoir (power hydraulic system) for my 1988 Vanden Plas?

Try Motorcars Incorporated in Plainville, CT.
They have several parts cars on their lot and may be able to grab one for you.
800-899-1055
Ask for Dean, tell him Dave Brill sent ya.

Talk soon,

Or contact JagtechOhio here on the forum, he has a lot of good used parts kicking around...

Thanks all,
From forum suggestions that the reservoir is most likely not the source of the leak, and the risk of purchasing a used reservoir that may leak or leak in the future I have decided to seal every orifice on the reservoir with DAP aquarium sealant and fill it to see if it's the source of the leak.

A little late , but I just pulled mine when I converted to vacuum brake assist.
do you still need one?