I would take photos with rulers in them for size reference.
Then send an email to some of the various RV salvage yards with the information about the make, vintage and the image.
Those junk yards often have boxes sitting with assorted bits and pieces in them. No doubt one of the RV salvage yards will have a grate or two on hand. That brand of stove was used by several makes including airstream, boler
and Serro Scotty.
It is a small enough part to be easy to box up and ship.