Walmart offers the RPS Products BestAir Water Pad Humidifier Replacement Filter, model no. A35, for $4.66. (Amazon charges the same with free shipping when you check out via Subscribe & Save.) Opt for in-store pickup to avoid the $5.99 shipping fee. That's the lowest price we could find by $3. It's compatible with select Aprilaire, Honeywell, Carrier, Bryant, Totaline, and Lennox humidifiers.