Gday.
you have obviously tried to source one through yamaha????
If you did and had no luck, try these guys:
http://www.pitmansa.com.au/motorbike...parts_list.htm
This is my local Yamaha dealer in Adelaide, South Australia.
A few years back i rebuilt a 79 model DT250 and it was amazing the sort of spares that these guys had sitting on the shelf for such an old bike.
It might pay to drop them an e-mail and just ask. They have all sorts of old and obscure parts in their store.
Good luck!
Dan.