I thought that the cover plates where only for the regular new 3DS, not the XL version.
Search Google "nintendo 3ds cover plates"
I think he means an actual case to carry the 3DS in
To answer your question, OP, I'm not sure where to find one. I have a big case that was meant for the original NDS and still fits a New3DS XL, the charger, and still space for things like the screwdriver I carry around for the 3DS.
The only issue is, I have no idea where I got it because it was given to me by neighbors a few years ago
