The Nuki can also be used without the so-called priority function (= lock can be turned from the outside even if a key is inserted inside). Of course, this is only recommended if there is a second entrance. In a flat with only one entrance door, there is a risk that you lock yourself out if the Nuki does not work anymore.
I ordered a locking cylinder from
https://www.fand.ch. I was able to enter all the details in the form that is available for download. The cylinder was then delivered directly by DormaKaba. But the price is steep and higher than the price for the Nuki.
I recommend contacting a locksmith of your choice and getting a brief consultation. No problem at all!