Here's my little odyssey to get my hands on this hose fitting:
My first stop was ATU. I thought, since they carry all brands, they would have the widest selection. I was directed to the workshop and told to ask the master mechanic. After a long search, he gave me the two smallest (brake bleeding) nipples he had, free of charge! Unfortunately, when I tried them later, they turned out to be too thick, but not by much. I think they could have been made to fit the aluminum with a little force...
2. I visited the Opel parts dealer for my second attempt. He disappeared into the warehouse with my ATU part and, much later, informed me that he didn't have a smaller one either.
3. Next, I went to a small, independent repair shop whose employees I know well. However, they were also unable to help me and assured me that there's nothing smaller available in the automotive parts sector.
4. I went to the Bosch & Hydraulics service center. The same situation as before: they can handle larger sizes, but not smaller ones. However, I bought 3 meters of 'fuel hose' there – a black rubber hose with a fabric outer layer – for the incredibly cheap price of 3.62 euros. I don't like the solution using clear hardware store hose...
5. I had almost given up hope when I walked into a large toy store (Spiel-Pfennig in Bayreuth). Here too, they initially said they didn't have it, but when I specifically asked about the 'nipple' (referring to the part), they showed me a shelf where I found what I was looking for. They had everything: M3, M4, and even my long-awaited M5! And all for €0.98.