NavList:

Hello, Steven Gardiner.

Welcome aboard!

You say you're looking for a replacement horizon mirror assembly for your USN Mk II sextant. So not just the mirror, right? You say the "mirror assembly" so I assume you mean you want to replace the frame that the mirror sits in, too. If there was some sort of clearinghouse for parts... "I want..." on one side, and "I have..." on the other, this request could probably be fulfilled quickly. Unfortunately, getting the word out that such a clearinghouse for sextant parts would be more expensive (advertising) than any potential fees, so for sextant parts, it's probably not possible. But all is not lost...

There are people who routinely restore old sextants who have been at it for years. Bill Morris and Ridge White for example. One of them may well have the components you're looking for.

If you still have no luck, perhaps the best way to get parts for an old sextant, is to buy another old sextant. I'm not kidding here. There's a key formula at work:
   1 + 1 = 1.
One damaged sextant plus another damaged sextant yields one good sextant :). If you look on ebay, you will occasionally see listings for sextants that are missing many components. If you can find one that's missing its index arm, for example, but still has its horizon mirror assembly, you could buy that whole (broken) sextant just to get the few parts you need. The skeleton of the old sextant that's left might make a nice wall decoration. 

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA