selfbus.org

I wonder where to get the red/black KNX bus connectors used in for example the Eingangsmodul ARM?

The red and black are the original one.
I use orange and blue. The number at Reichelt as follow:
Wago 243-722 (blue)
Wago 243-723 (orange)
Wago 243-131 (pin)

Red/black is Wago 243-211, grey/yellow is Wago 243-212. If you google those numbers, or just "Wago KNX", you'll get plenty of sources.