Craft stores, on line, like Amazon.
Any craft and hobby store, paint stores, many on line retailers.
Hobby Lobby, Michaels, WalMart, and online: EBay, Etsy, Amazon, Scrappin Along Stencils, thestencilsmith, and many more! I also cut my own stencils using my Cricut machine and Stencil Ease clear stencil material found on Amazon.
Craft Shops - eBay - Amazon etc.
On eBay or Amazon or Craft stores.