The filter for the rear heater in my bus disintegrated. foam sandwiched between two layers of mesh steel. HD did not have a size that would fit, but they do have universal "vent" filter material (white fibrous material). I cut it to the right size and covered the old filter's frame with it. Worked great for that rear heater.
however, the front dash heater's filter slides into place (find it behind a long-narrow cover above the front stairwell) and that is harder to fix.
perhaps I'll try stapling the new filter material to the old frame, but I still wonder if there will be clearance to re-insert it into its slot.
A new factory filter will likely need to be ordered from BlueBird