Copper crush washers

For my Model 18 I need to find a supplier for the exhaust pipe to head joint sealing washer.  I've found that they are called Copper Crush Washers but most sites usually only give the internal diameter of the washer, and I need the outer diameter dimension.  This is because the exhaust nut will only allow a 50mm washer to fit in.

I've tried an ES2 and International washer but they are 2" OD and will not fit.  An aerospace company in Okehampton used to stock them, but not any more.

Any suggestions please?