Mylar drafting material? $$$$

You can trace the outline on plain wrapping, banner, or freezer paper and cut out the dress silhouette to use as a test. Cheaper and easier to find.

You can buy that plastic at any quilt shop or a Joann Fabrics.

You might try using the plastic from 1 of those clear report folders...

Office supply stores will give you lots of options, since you need just a small amount. And what I think Lazy Gardens suggested will help the eye to NOT see what will not be there ---- and from what I know about drawing small things --- may be more effective.