Hello, as the other person said in his comment, just type 915 tkl key caps on Google and sites of Chinese origin offer all sorts of keys adapted to this keyboard at OK prices, for my part I found replacements for less than 10.-.

Hey, I have exactly the same problem as you. Unfortunately, Logitech itself doesn't offer any spare parts. But you can find enough suppliers via Google (I won't give any links, I don't know if digitec would allow this). I just had a look with "915 tkl keycaps". I even stumbled across an American site where you could order individual caps for an exorbitant price... There seems to be no other way, because as far as I know digitec doesn't have any of these third-party suppliers with matching caps in their programme. I played the same game with the previous keyboard and ordered a complete set from Asia. That actually worked out well.