Where do I find a brick stencil?

Are you looking to stencil onto brick or to make faux bricks. If the latter, You don't really need a stencil but masking tape, about 1/4" thick.Tape can be found in some hardware stores in the grouting department (used to give a sharp, clean edge), or in the quilting section of fabric stores (quilting guide). Paint the area in the colour you wish to have the grout of the brick. Once completely dry, lightly draw where you want the grout (tick marks at edges). Tape in your lines. Pick you paint colours. Use an aluminum pie plate and pour puddles of paint. With a sea sponge, pounce the paint to mimic the brick colour. When dry. Carefully remove the tape slowly.

Hi ! You can try Amazon , you will have several styles and sizes .

Any craft or hobby shop, many on line retailers.

I would look on Amazon or Etsy!

Here you go, Amazon has tons of them and prices are reasonable as well. https://amzn.to/2QwqoRS