Holmes has discontinued the HAPF9 HEPA Filter and lists no replacement.
The Holmes HAPF9, 9000494 3 Layer Replacement Filter fits Holmes models HAP100, HAP-100, HAP100N, HAP-100N, HAP1004, and HAP1004. This filter can also fit Norelco CA220, and HB1920, as well as Conair Models E3, and E3AN.
The HAPF-9 needs to be changed every 2-3 months. Filter measures 5 inches across. Price is for 1 filter.