Most places seem to be either old school "request a quote" businesses, medical industry only, special order (Bunnings) or super overpriced panels made for specific air filters.
I've heard furnace filters could be an option but they seem to find at a Merv of 17+ (equivalent to a HEPA filter).
So does anyone have a good supplier for simple square or rectangle HEPA filter panels at a reasonable price? The mount isn't important as it's for a DIY project.