Sometimes they use Kaplon or Mylar cut by laser. Not sure of benefits of one over the other. But the cheap one I saw was a polyethylene synthetic paper, which apparently works also and was cut by regular vinyl cutter.
I've seen Kaplon/Mylar ones for $35 on some US site, and the synthetic paper one was $10.
Kapton has a very high melting point and as a result when cut with a laser it doesn't warp as easily, making it a better stencil. I have reballed BGAs using a Kapton stencil to keep the balls in place when applying heat.