You can get scalpel blades online through ebay, etc.
If your design is for a set of stencils you want to use repeatedly, then you might try a similar method to the one listed already.
Keep in mind that it's much easier for black to cover other colors than it is for other colors to cover black, especially if you are airbrushing those colors.
First, draw out your design or have the program of your choice do it. I did mine in Illustrator, this is a quick and dirty version based upon the original file posted. i use triangles for registration marks, they are easy for the plotter to cut and give me easy reference points to align my artwork. I place them far enough away from the design area so that I can put down a patch of blue, painters tape in those spots. This lets me paint the triangles at the same time as the rest of the areas, letting me make sure that everything lines up perfectly, and without effecting how the stencils will lay against the substrate. I've shown two different versions for the black areas of the paint job, one is a complete fill that I would place the red stencil over. I'd fill that area with white (over the black parts) and after that dried I'd shoot the red. The other option is the version on the left, it has just the outlines but with bridges to support the interior panels/masks so that they can fill with the red. Once I'm finished with the painting, I can peel away my bits of painters tape that cover the registration mark areas.