I am confused. Are you talking about an AC/Heater filter??? I would just go to Lowes or Home Depot, where they have a large selection, and if you buy several, it is cheaper. Hepa filters are more expensive. Make it an absolute rule to change your filter every 30 days! Doing that will save you a lot of time and eliminate much of the dust and allergens. You could also look online, but personally I am wary of the many and outrageous claims people make about their products online. Oh, and make SURE you buy the correct size!