You can purchase replacement nozzles/filters for the Kato from the Moondrop Official Store on Aliexpress. See:

https://www.aliexpress.us/item/3256803559403072.html

Alternatively, the 4.0mm diameters dust filters from either of the following two Aliexpress stores should work just fine since the Kato filters are merely dust and earwax filters and are not meant to significantly affect tuning:

https://www.aliexpress.us/item/3256803958562978.html

https://www.aliexpress.us/item/3256805399657415.html