Please be advised that the filter media in your Holstr 3R is washable. If you need to replace it entirely, 1/2″ media works well. In cooler climates during winter you can step it up to 3/4″ to keep out any fine snow blowing around. We recommend the MERV 4 rating for your replacement filter. The filter media that came with your Holstr 3R was manufactured by PolyKlean. It should be available at your nearest Grainger’s.