Is it really broken or in need of a rebuild? Mine leaks and just needed a rebuild. It's a Girling which was the go to stuff in the old days and now owned and apparently killed by TRW. I found a place in the UK last fall and bought a rebuild kit. All is well now.
If you really need to replace the whole thing take good measurements, including the rod length and look around. They are everywhere in the Hot Rod world.
UK supplier has the Girling too.
Lockheed and Girling Clutch Slave Cylinders for classic and vintage vehicles
Tim