First you should know that there are only a few reliable brands.
A good mask should filter pm2.5 effectively (100% filter). And it should be tight on your face and let little air come through some gap between your face and the mask!
Among the reliable brands I tried Totobobo, Respro and Vogmask.
Totobobo is good but not convenient.
It fits your face well even though you as a foreigner have a big nose. It also allows you to breath well and talk. Though wearing Totobobo you face will be very quickly drenched in sweat! In fact Totobobo is made of not made of fabric thus water vapor cover the mask and your face. This is really really annoying!
Respro is good but not convenient.
As for Respro, it will leave heavy mark on your face as it is really tight. It also doesn't allow you to breath well or to talk.
Vogmask is really something else.
It allows you to breath and talk. It is not too tight. It fits the face well even though I have a big nose.
To buy Vogmask it costs a little more than 200rmb and you can order from their website (from their Taobao shop). They deliver to your home in a few days.