E-Z cut Plastic is my choice stencil material. Easy to cut, easy to use, and can be purchased in large sheet sizes.

From the E-Z Cut Plastic page:
Wonderful clear materials for cutting your own stencils! Nicknamed "cuts like butter", E-Z Cut Plastic is very easy to cut with a stencil burner or X-acto knife. It works incredibly well with the stencil burners, and doesn't get any frustrating ridges, as other materials do.

Available in 2 widths and thicknesses:
-Original E-Z Cut Plastic: 24" wide, 6mil; cut into any length you desire up to 200 feet!
-Heavy E-Z Cut Plastic: 36" wide, 10 mil; cut into any length you desire up to 100 feet!

Perfect for stencillers - this EZ cut plastic goes is a must have. Easily cut your stencils out and apply to desired surface and voila! Art abounds!