I had my master cylinder overhauled, but the brake place couldn't supply a new rubber gasket that sits between it and the booster.  I had a scrounge around a hardware store but didn't find anything suitable.  Anybody know where to find one?  They have an internal diameter of about 40mm, and I'm guessing about 6mm thickness.
 
Thanks!

Not sure where to get one but here is a pic of what you want.
 
Let me know if you need a better pic.
 
Cheers,
 
Matt
 

Thanks Matt.  I've been to 4 different places (5 if you include the joint that did the master cylinder) and nobody carries a square cut rubber seal off any description.  I guess I shouldn't be surprised by this, but I am.

You may just need to get a new (old) booster as well.
 
I have one here in Melb you can have if you want.
 
Cheers,
 
Matt

I have a rubber for the booster.

Thanks everyone; yesterday I ended up using a couple of O-rings, which although not as wide as the original seal, will hopefully do the trick.  Frankly, I'm not entirely sure why the seal is there anyway, especially seeing I didn't have any trouble with the old one, which looks to have dried out, shrunk and flattened about 20 years ago.