You can get the filters at Galaxus, for example. It is important to know that the filters can be used longer than the devices suggest. The first filter in the filter retains coarse dirt such as dust or hair, which can be vacuumed up with a hoover. Then put it back in the Xiaomi air purifier and simply perform a filter reset. This way, the filters can be used for a very long time. Because here in Switzerland, the air pollution is not comparable with countries in Asia.

https://www.galaxus.ch/search

The grey filter is the best, it is a HEPA H13 filter, but it would be much cheaper at Amazon.de including shipping :).