Dearborn Supply via Amazon offers Prime members the Aprilaire 10 Water Panel 4-Pack
with free shipping
. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $7.17.) That's the lowest total price we could find for this quantity by $14. These humidifier water panels fit Aprilaire models 110, 220, 500, 550, and 558.