For durable stencils I use the clear plastic that makes the "window" in the box for kids toys. (my most common source) They are usually solvent proof so you can easily clean them when the get yucky with too much paint.
Need fancier stencils? Create the desired artwork in Adobe Illustrator or Coreldraw. Save or export as an .eps file and take it to a sign shop that does cut vinyl. Some shops have a minimum of around $50. Shop around, I go to a guy who has a $20 min. These type stencils are pretty much one use type so it could get expensive quickly.
To use the toy-box-window for fancy, just print the above mentioned computer art in black & white only. (No halftones) Then lay the clear plastic over the printout and cut carefully with an Xacto #11 blade.

Hope that helps,
Dr. Deltron