Few of these ever come up for sale.
If you have no luck on the forum, you could put some feelers out at dedicated 10 metre target clubs.
It is those shooters, that may be able to help. Someone may have one that they no longer use after moving onto pcp’s, years ago.
I remember a chap from a target club in Harpenden, saying he had one and had resealed it.
The other cohort for this pistol would be collectors. But it could be harder to get them to part with one.
Also, definitely try the next Kempton.
I was lucky enough to pick up an excellent condition and fully functional FWB 102
in a bespoke fitted wooden case, for a great price, a few Sundays ( although the event was temporarily moved from Kempton)
Good luck with your search. The k58 is great match pistol and definitely worth searching for.