Brake Booster

Another question for the day!

The brake booster in my car is shagged. Are boosters available? Kits to rebuild them? Haven't had much luck in OZ finding anyone willing to source the bits I need - or even interested enough to play around with it.

Are they available anywhere? Has anyone substituted anything from another car? I've wandered around the wrecking yards but its horribly confusing - and most more modern cars have MUCH larger diameter units that the one in the Bagheera. Having a basic understanding of how they work (but no practical experience) I'm guessing that the diameter of the diaphram is the critical aspect?

Thanks in advance, once again!