Try giving Granby Yamaha a try.

http://granbymotors.co.uk/motorcycle-spare-parts

They got all of the plastics for my 1985 XT600Z Tenere. They have access to both the European & Japanese stores and will take the time to search for you.
I also get 10% discount from them as a member of the Aircooled RD club, so it's worth asking. However the discount only applies to stock items, I didn't get discount on my plastics as they were special order....and cost �300!