Stencil Revolution has a huge assortment of Christmas Tree stencils... I hope this helps!
Here are some free Christmas tree templates and stencils: https://www.thesprucecrafts.com/free-christmas-tree-templates-1356277
Hi Robin: Michael's, Hobby Lobby, and most craft stores carry them. Or, you can make your own. Just draw your tree on a piece of cardboard, cut it out and use the opening for your stencil. Good luck
I am not sure how big you want your stencil but there are some cute ones on Amazon too.
I'm sure you could get one on Etsy.com