Hi Anparkinson,
I was in the same boat a year ago. I got nos 2 and 3 from D Silver. About a month later I got no 4 from Econohonda in NewZealand.(All brand new)
I laboured for ages looking for no 1 and eventually picked up a brand new one on...... guess what.......
ebay.co.uk
Keep up the search using the number stamped on the exhaust and the two renditions of the Honda parts numbers - 3 in total per exhaust.
My Bike is 1978, unregistered with one owner and 32 miles. I am just about to start the rebuild.
Best of luck,
Gary