Weihrauch did have a diopter set - usually on the HW55, but very rare. I have one in my parts box but im kinda holding onto it in case I come accross an HW55 to mount it on. I believe they are 13mm dovetail too, not the modern 11mm.

There is a cheaply made diopter available under a few brands - Air Arms used ot market it, Gamo, Crosman, BSA, basically a copy of the old Feinwerkbau 300s unit. I dont think the adjustments are as good as match grade quality but probably ok for entry level.

Im not sure where to get one though, perhaps a UK based retailer?

otherwise you could try caling around smallbore clubs to see if there are any units spare that could be sold, or try ebay / egun for an old Feinwerkbau unit.