I have seen several sell on Gunbroker over the last year of so. I have tried to buy a couple of smallbore Merkels but they went too high.

Here is a Fortuna listed right now that I believe falls in the same category. The seller claims it was sold by Merkel too. It is a higher grade gun.

https://www.gunbroker.com/item/1048141614

Here's a Merkel:

https://www.gunbroker.com/item/1036832245

This 20 ga is about as good as the GDR guns get.

There are numerous Merkel guns plus Fortuna and Simpson guns for sale right now.