You should first confirm the air purifier model name to find which filters are compatible with your unit.
The Serial Number and the MAC address appear on two different labels underneath the device.
If your air purifier is connected to the Blueair app you can additionally view the MAC address by selecting air purifier name and navigating to Product Settings › Product Information.
Once you know your model number, you can go to https://www.blueair.com/us/air-purifier-filters. Scroll down and select the model you have. All compatible filters and pre-filters will then be displayed. You can then click and order your filter as needed.