Mylar isn’t so good for bumper stickers - for one thing is isn’t nearly as flexible as vinyl, and for another it rarely comes with adhesive backing.
Other than some specialty reflective and retroreflective films, I haven’t seen much adhesive film that wasn’t vinyl.
For stencils (face paint, airbrush, decorative) you want 0.003 or 0.005 mylar (dura-lar at the art store). That can be cut on laser cutters and most hobby paper/vinyl cutters (Silhouette, Cricut, etc.).