But I seriously wanted to make some homemade envelopes in that style so I got to thinking...and I decided to make my own template. First, I chose one of my Japanese envelopes to use as my pattern.
Next, I carefully pulled the glued flaps apart so it would lay flat, traced around it onto a clear plastic folder I bought at Staples for $0.99, and cut it out.
(The slideshow below has more pics showing the process.)
Updated 12/18/12: link to printable template of a Japanese-style envelope.