You can find the filters f.e. here: https://www.3m.com.hk/3M/en_HK/filtr...4529192&rt=rud
But I have to admit: they claim that they take out 40 times more PM2.5 than general air conditioner's plastic screen filter. That is a bit of a dubious claim since I think that those plastic screens take out 0% of PM 2.5, so 40 times 0% is still 0%. Interesting discussion though, I think I might want to look a bit more closely into this issue because the air quality is really appalling lately (it's my first winter here in HK, so I am new to the issue).