Paul,
Originals may also have been used on DINO's.... but you'd probably never find those parts either.....
Rebuild your M/C! Have it sleeved in bronze by Sierra Classics (????? I hate when I forget stuff....) and the just get new 7/8" rubber donut seals from any Alpha dealer. I think they are rear brake cylinder seals in them...
This way, you'll never have to worry about a rusty bore, and you will probably be able to get by with just replacing the seals next time!!!
I did this to my original, and it's holding up just fine!!
Steve