I think you also need to consider your process and how you cut the stencils if calling a stencil food grade. I know that Designer Stencils cleans its cutting machinery/stencils in such a way that no harmful materials get on/remain on the stencils.
Global Sugar Art and Country Kitchen Sweet Art sell food-grade acetate in different forms. Perhaps you can check out those brands and place calls to them to determine how they qualify themselves as food grade? What materials and processes do they use? Who certifies this designation - themselves or some external, independent body? Going directly to the source is probably going to tell you a lot.