This is the single Brno trigger you are looking for. I had a ZKK with the set trigger as you have and while it was nice to use in terms of let off I found it too far forward in the trigger bow to be comfortable for my length of pull. A friend also with a ZKK had the spare single trigger, they came in a little green cloth bag with a small flat spring to prevent the trigger rattling around when installed in place of the set trigger. I always intended using his trigger to copy and make one for my ZKK but never got around to it before selling the rifle. I don't know that anyone will part with one of these sort after triggers unless they have sold the rifle without the replacement trigger.

Be careful if adjusting the let off of your set trigger (via the screw accessed just behind the rear bridge when the bolt is out)as there is a point where the cocking piece sear will not catch the trigger sear and the firing pin will fall if operating and closing the bolt rapidly. While this may not normally discharge a cartridge it could do so if the bolt is slammed shut on a loaded round.