CC Return Air Filter Replacement

Has anyone found a source for replacing the foam filter that has a card board border around it? This filter is inside a metal return vent cover for a ducted return air system. Since we have dual air conditioners there are 4 return vents with filters. The card board border appears to keep the filter in place and not allowing it to moving around.


Tom
2016 Ford Power Stroke F350 Lariat Crew Cab Long Bed Dual Wheel
2015 Cedar Creek 38RD Hathaway Edition