You can hand-paint your own stenciled border. Use painter's tape to mark off straight edges; you'll find the tape prevents under-bleeding. Cut your stencil out of contact paper, then blot it on your t-shirt or sweatshirt. Doing this makes it less sticky, so when you peel it off, you won't damage the wall paint. Seal first with an acrylic matte medium, let dry, then dab on acrylic paint with a sponge brush. When dry, peel off for a perfect stencil every time.
For more on finishes, consider:
How To: Stencil a Floor
Stenciled Floors: The Best of Today's Designs
8 Ways to Age, Distress, or Add Shine to Your Next Project