I've always used Filterbuy for my air filters. They fit perfectly, their packaging is high quality - boxed well and sealed in plastic - and the difference in the air when you get that new filter in - amazing. I know they got what I need so I'm always coming back - and its shipped right to me. Easy. Fast.
Not quite time to replace my existing 6 month Honeywell filter yet but it is back to 4+"(unlike my Honeywell which went to 3") and also looks to be better "quality assembly" than my previous filter suppliers. They had issues with getting all the ends of the pleats glued to the sides.