Walmart offers the 3M Filtrete Air Micro Particle Reduction and Furnace Filter 3-Pack for $15.88. Opt for in-store pickup to avoid the $5.99 shipping charge. That's $11 off list and the lowest price we could find. (For further comparison, Walmart otherwise charges $9 for just one filter.) They're available in select sizes from 12x24x1 to 20x25x1.