You may be able to pick a tatty peep sight which includes the part you want on that German site. They do turn up quite often.
There is a half decent complete one with a buy it now option for 90 Euro at the moment, but I've seen tattier ones go for much less especially if you are bidding. You may get lucky.
I find it's easier to use the German language version and type Diana in the search box and scroll through the results using the pics. hth.