If your device is connected on the app, you can simply click "Replace My Filter" > "Order Filter" and select "One Time Purchase" or "Sign up / Manage Filter Subscription" which will take you straight to the correct filter on blueair.com.
If your device is not connected on the app, you can visit our filter page here https://www.blueair.com/us/air-purifier-filters and select Classic Pro as the family and your model as the series.