I bought mine from Z1 Enterprises.
http://www.z1enterprises.com/detail.aspx?ID=2653 It returned my master cylinder to specification, and Z1 is a first class operation.
Having said that, SOHC master cylinders are pieces of crap, IMO, and rebuilding them is a real pain. Do a search for "master cylinder circlip" for details. I am never rebuilding another one becasue the results just aren't worth it. For the price of the rebuild kit you can find a used master from a modern bike and be far ahead of the game.
IMHO, as always.