I've been ordering mine from Amazon, last time was in 2014 a 3 pack of Merv 13, 16 X 20 X 4 with shipping was $38. The photo shows 2, but I got 3. I replace mine once a year, that's what the Munn's tech told me to do back in 2011. They've gone up to $42.99 for 3 pack, same ones I got because I searched my Amazon history, here's a link.
Amazon.com: US Home Filter SC80-16X20X4 MERV 13 Pleated Air Filter (Pack of 3), 16" x 20" x 4": Home Improvement
