I've never used a stencil with my airbrush set up (not yet that is). I really don't know much about them but here is a link where one can be ordered...
http://www.sugarcraft.com/catalog/airbrush/stencils.htm
They have pictures of stencils they sale that may give you some idea of how it's done ... sorry I couldn't offer more advice on this .. maybe someone else that knows more about stencils will see this post and offer some more comments/help!
Hi mary-ann:
I´ve made some stencils for cookies. I think it could be used for cakes.
I use a simple cutter and carefuly cut on transparent paper which is little heavy.(i hope this is the word).
you can find them at stationery store.
I post the picture of the cutter I use. Unfortunatly I don´t know the translation exact for the paper 
I hope help you.
Saborita
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