Hi Andrew,
Many people ask me that one and flasher switches are indeed very rare.
I had four 1966 ones and gave them to friends a couple of years ago. Andy Booth and Alistair got two of them.
The last one that found and recommended to a friend went to him on eBay for £350. I'm afraid that it's a case of keep searching the web until one surfaces and hopefully not as expensive.
It's a shame but I don't think that I will be doing another car project for a while so will pass on your kind offer this time.
All the very best,