MMW-
Door panels appear to be caught up in the supply chain disruption.
Try PPL Motorhomes. They are a big distributor. They appear to have stock of the plastic N841 lower panels (link
here). Please confirm for yourself, but I believe the N821 and N841 use the same size panels.
Last year I replaced the wooden door panels on our N841 with magnetic stainless steel panels I bought online. Except for the fact that they can corrode, the panels have worked out well. I can now hang calendars and magnetic items on the doors.