Aprilaire #10 - Compatible Replacement - Water Pad Humidifier Filter
Tier1 2-Pack Replacement Humidifier Filter in the Humidifier Filters department at Lowes.com
Amazon.com: Tier1 Humidifier Filter Replacement for Water Panel 10 Aprilaire Models 110, 220, 500, 550, 558 - Improves Air Quality in Homes and Offices - (2 Pack) : Home & Kitchen