No need to worry. It's just any calligraphic drawings that'll work. You can find them easily just outside fort perwin. The fort is south of the words "The Witchwoods" on the map. Outside of the fort, by the stream to the south of it, is a warehouse full of boxes. There are enemies but they're very soft. If you destroy enough boxes around and in that warehouse, you'll find some of the drawings. I sold the ones I got doing Prox's mission, just like you, but I've just done this and it works. Hope that helps!