I couldn't find any ready made either, so I make my own. Struggled making the first one and now
I use it as a pattern. Made it for the 9x13 pan and also spongecake tube pan (just a circle for the bottom of pan with a circle hole in the center). It really isnt that hard. I just fit the paper in the pan and ran my fingers around all the sides at the bottom of the pan, which kind of marks the size of the bottom. I then creased the paper along the lines and then cut out a wedge on each corner so that the parchment paper would fit the pan without any excess in the corners.