Hanging in front of me in the store were stencil blanks, the kind you use for cutting your own stencils. I bought some and brought them home only to realize they were not unlike the plastic the folders were made out of. The only difference is that the folders are thicker. Perfect for envelope templates, and very colorful!
I used a template from Mirkwood Designs for my pattern. Ruthann has templates for several different projects (like ATC envelopes, miniature envelopes, take out boxes, triptych cards, the list goes on).
I printed one out, cut it out, and cut my folder in the shape of the envelope. Then I used a sharpie marker to mark the folding lines on the plastic and write the directions down on it.
This is great to have around the house, and is also an excellent extra for mail swaps!
Mirkwood designs offers the patterns for free for personal use, but if you are able to please help her keep the site going with a donation. She is set up to receive donations through paypal, which makes it convenient (and safe). Just make sure to give credit where it is due (I put Ruthann's web site on this one since I will most likely add it to a trade envelope).
Ruthann also has a fun and informative blog/podcast. Stop by and check out her free projects and tutorials for paper crafts and fiber art (and the occasional receipe). Fun!