You can make your own self adhesive airbrush stencils or buy them ready to be used.
In the majority of cases I have seen, airbrushers make their own stencils and cut them out manually. Some use a stencil cutter.
I my own airbrush school moment in Zurich I used a repositionable spray adhesive (I used a 3M special glue ) on a...T-shirt surface!
It worked excellently.
But these stencils are mainly ment for tattooing and body art.
A nice place where buy them is this one.