Google onto the Shiva Kunal Import Company site and check out their gear. I picked up a nice adapter for my K1000 and I know they have a bunch of Nikon stuff.
Good Luck.
Mike
I have a few 'valuable' cameras and I have the caps, but in all the years I have shot, I have never once scratched a viewfinder lens, they are pretty well protected from damage unless your shooting in the desert with a strong wind blowing..
I got a cap for an N65 from B&H. I got it for doing auto exposures with a cable release, not for protecting the lens. I have also never had an issue with damage to the viewfinder.
Have you thought about asking the question on the Nikonians website? There is a thread there devoted to manual Nikons and a couple of knowledgeable contributors who are in the British Isles.