Reddit mentions of 5 ml Clear Acetate Roll 40" wide by 12 feet

I like using this kind of stuff. It allows you to make much bigger stencils and because it's clear you can trace other images right onto it pretty easy.

You can use things like cereal boxes, but just remember that over time they'll get soaked through with paint and whatnot. The edges may begin to wear and then your stencil will lose detail.