Not the cheapest deal, but $45 with free shipping is reasonable.
Three options are available:
Anti-bacterial Filter
https://www.kogan.com/au/buy/xiaomi-mi-antibacterial-filter-…
Effectively removes bacteria, dust mites and PM2.5 particles in the air
Specially designed antibacterial layer
Quadruple purification
Designed for Xiaomi Mi purifier, Xiaomi Mi purifier 2 and Xiaomi Mi purifier Pro
The Xiaomi Mi Antibacterial Filter for Air Purifier combines air cleaning and sterilising technologies that will help you say goodbye to allergy causing pollutants and bacteria and enjoy breathing clean air!
Anti-formaldehyde Filter
https://www.kogan.com/au/buy/xiaomi-mi-anti-formaldehyde-fil…
Unique bucket-shaped filter
3-layer filter design
Removes formaldehyde, PM2.5 and other pollutants from air
Filters out 99.99% of organic matter
Easy to install
The Xiaomi Mi Anti-formaldehyde Filter for Air Purifier consists of 3-layers: elementary filter cloth, efficient filter material and activated carbon filter. The first layer filters out hair, paper pieces, etc, while the second layer can effectively remove dust and pollen from air. The activated carbon filter is used to absorb other harmful substances from the air and gives you fresh and clean air to breathe in.
Xiaomi Mi HEPA Filter for Air Purifier
https://www.kogan.com/au/buy/xiaomi-mi-hepa-filter-air-purif…
Replacement Xiaomi Mi HEPA filter
Comprehensive 360° filtration capability
3-Stage purification with primary, true HEPA and activated carbon filters
Captures up to 99.97% of airborne particles
Activated carbon helps eliminate gases, smog and odours
High performance 380m3/h CADR
Regularly replace your filter every 6 months
Compatible with Xiaomi Mi Air Purifiers
Send allergens packing and enjoy breathing cleaner, fresher air for longer by regularly replacing the filter in your air purifier with the Xiaomi Mi HEPA Filter, purifying the air in your home to help protect you and your family from respiratory stress triggers.
A good comparison article: https://jumpadevice.com/2019/01/16/choose-the-correct-filter…
This link has a comparison at the bottom too. Grey one is H13 vs H11 for the green and purple ones.
https://www.mi-store.com.au/when-to-change-air-filter
Also the green one is $49?