As far as I know, I need a 5V/2A power charger. But where can I buy this (in germany)? Is it possible to use a 5.1V/3A power supply, that is originaly for raspberrypi 4 or a 5.1V/2.5A originally for raspberrypi 3 B+ ?
Sure, 5V/2A are the minimum requirements, all chargers >=2A should be fine.
Ok, thank you very much 