What's a good plastic for self-cutting solder paste stencils on a laser?
As you noted, at Pololu, we use Mylar for our Laser-Cut Mylar SMT Stencil service. It's not that hard to find Mylar films: McMaster has a bunch of them. Take a look at McMaster part number 8567K32. (Sorry for not linking, McMaster is hard to link to.) The main trick is carefully tuning the cutting settings and design to account for the quirks of your cutting process. We use internal computer programs we've written to modify the designs to make the cuts turn out well in our process.