Hardware stores, such as Ace Hardware and The Home Depot, and general retailers, such as Amazon.com, carry furnace filters. Furnace filters should match the airflow recommendation from the manufacturer to ensure a long lifespan for the furnace motor and to be energy efficient.
Common furnace filters are made of fiberglass or polyester. These are disposable and good for catching large particles but generally need to be replaced often. Reusable filters only need to be replaced every few years but need to be cleaned regularly. High-efficiency filters are good for catching small particles but are generally more expensive than fiberglass. High-efficiency filters are typically recommended for households with a high amount of air pollutants, such as those in which pets or smokers reside.