I'm not sure if this is the right chat room but it's paint orientated.
On eBay, there are weathering airbrush stencils by Llang, predominately for 1/35 afv's.
Just to inform you, these are not made from brass or any other metal but laser cut wood or card, I can't quit tell exactly which.
I just thought I'd let you know if you already didn't. I'm not sure if they need to be coated/varnished so the paint won't seep in and expand the material.