Just thought I'd pass this along... I was not able to source new exhaust crush gaskets locally for my '76 so I did some searching and found some at Aircraft Spruce - aircraftspruce.com/catalog/eppages/an900.php
I bought 4 x AN900-20 / MS35769-31 copper crush gaskets for $0.70CDN each (thats a little better than the $4.30 ea. I have paid for the Honda part). They have been on the bike for about 500Kms now and so far, so good.
Aircraft Spruce has online presence in Canada and the US and is also a good source for O-rings.
Hope this is useful!