They are very hard to get hold of and cost alot. If you are not trying to make it 'as new' my advice is to get a Weihrauch foresight and file the dovetail clamp out so it fits the Feinwerkbau barrel, and use an HW77 rearsight which fits without modification on to the scope-rail. The HW foresight has easily obtainable elements which are really good for all kinds of shooting too, while fitting the HW77 rearsight gives you a longer sightbase and therefore a more accurate setup.
If you want a nice peepsight, one of the nicest is the Beeman Sport Aperture which you can get off American amazon.
http://www.amazon.com/Beeman-Sport-A...2853620&sr=8-2
for about £40. It is beautifully made and complements the engineering quality of the Feinwerkbau far better than the original plastic rearsight.