The first furnace my folks had in their house was an odd size, always a PITA to find filters (usually involving a trip to either the other side of town or state, never got them at the same place twice.)
When it finally gave up the ghost one thing dad insisted on was the filter be a size that was in common use, could be ready found not at any hardware/big box store but all of them, Lowes, Menards, HD, Ace, True Value.
I remember Dad telling the guy, "If I go to the hardware store and the don't carry them, I will personally come to your house and drag you to every store until we find them and make you buy every single one they have so that I have enough stock to never run out."
Had the installer been a dog he would have had his tail between his legs. DO NOT PISS OFF DAD. (Trust me on that.)