Geoff at Alretta Truck Parts would be my first stop. I don't know if he'll sell you a rebuild kit or not but he will most likely have the hydrovac in stock. Carquest and Napa may have the whole unit too, if you can find someone who know how to look.
Oh, and Geoff is really a phone type guy. My emails went unanswered for a couple of days till he finally checked it. He recommended calling in the future.

http://www.alretta.com


1947, 1950, & 1952 Chevrolet 1/2 ton
1952 GMC 450 series Cab Over Engine
1946 Chevrolet 2 Ton