Tried a couple of places over here
www.grintriumph.com
which shows various styes of exhausts for triumphs from that year.
and also
www.feked.com who advertise as classic bike experts,
This is an copy from their web page - know its not a thruxton but they have an enquiry section which might help.
Triumph Exhaust Pipe Bonneville Thruxton 650cc 1967 on
Price (+vat): £114.50
fkt-ae
These will fit the Triumph Models: Bonneville Thruxton 650cc 1967 on Please specify which model when ordering. A replica of the originals, but these are made from a heavier guage steel than most of the other manufactured pipes you can purchase. This means that they last longer, and are less susceptible to light damage and colouring. These exhausts are made here in Britain, as much as possible the welds are levelled and not visible to the naked eye after chroming. The silencers are fitted with baffles where required, creating a superb sound that you would expect from motorcycles of this ilk.
Related items:
Triumph Exhaust Silencer Bonneville Thruxton 650cc 1967 on £72.95
Exhaust Clamp Chrome £3.50
more info...
Triumph Exhaust Pipe TR6, T120, unit 650cc
Price (+vat): £99.50
These will fit the Triumph Models: TR6, T120, unit 650cc
another place is on your side of the water which makes me think you've probably tried it
www.klemptsbritishparts.com
Unfortunately I can't vouch for any of these but i think I've heard of grintriumph before
Good luck