A half size sheet pan is exactly 13" (by 18"). See attached. That might work, but sometimes they measure differently from inside to outside so I recommend taking a tape measure to the store.
Another idea (cheap and disposable) is oven liners. not sure why these show as $36.80 on the Walmart site. They are a couple of dollars for a 2 pack.
+1 to Food Service Warehouse. I use them for all kinds of crazy stuff. FYI, their shipping rates are a better value when ordering multiple items at once.