Is there such a thing and where do I find?

You might find something here:

https://www.google.com/search?q=henna+stencil&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Of course most/all are not made for cakes, but if they're clean and the amount of time they would touch the cake is short, I don't think it would matter.