Check out these on Amazon: https://amzn.to/2VC7qz8
Any place that sells stencils. Or go to the post you were viewing and ask there. Craft and hobby shops and many on line retailers.
Karen: Etsy is always fun!
Hi, https://www.royaldesignstudio.com should have one!
Here's some options from Cutting Edge Stencils, they're a great company I've used several times https://www.cuttingedgestencils.com/catalogsearch/result/?q=birch