I managed to find commercial quality aluminum cookie sheets to fit my wall oven. YAY! They're NordicWare, and are square (another plus, as my oven measures 18 inches by 18 inches from wall to wall). Best part is that they didn't cost a mint. $9.99 each. Not bad! The size etched on each sheet says 13 by 14 inches, but including the lip the actual measurement is 16 by 14 inches. No matter. It fits!
You have to know what the rules are before you can successfully break them.