Hi Freon, I will look for them also, I found a place called HVAC something.
14 x 20 grlles are about $22 each I would need 6, they are called TRUaire 190s, also I found the MBR 600 - MERV 8 filtes for the grilles.
They will delever to Canada, perhaps the place you mention I can purchase the filters with the grilles, without the filters they are usless.
So I talked to my wife and she thinks this is a lot of work to retrofit the grilles.
I can see that more filter area means more air through the furnace better distribution of air I guess.
Makes sense that the furnace filter can only flow 800 CFM when the blower is capable of much more. I could see premature failure of the blower motor, the strain on it.
but what are the true benefits