My ventilation system uses what the vendor has called a "Simx G4 cartridge filter". I think its the Simx DCT2222 from this page. Basically, it's a cardboard box with a filter glued in, plastic panels with ducts attach to each side of the filter. Pics below.

Does anyone know where to source this cartridge filter? The Simx website "find a supplier" is terrible. I was told "an electrical wholesaler". I don't just want to order that given I've taken a guess about the part number, and getting it down from the ceiling would be a hassle.

I think this filter is good for a bit longer - they start out bright white and get replaced when you think it needs to be done. It draws air in directly from outside, which is apparently non-standard, but our roof space is horribly dirty / smells and has rodent droppings, rodent poison, etc.