The PaperSource sells templates along with the lining. You can use the score board to make your own, especially if you are making custom sizes, just know that odd sizes cost more to send through USPS. There are dies out there that also cut the envelopes if you are looking for standard sizes.
There are lots of other videos too, but I liked these because they were step by step instructions.
I have seen flexible plastic envelope templates sold on TV shopping networks, but I haven't found them in any of the craft stores I shop. I was very dismayed by that when I needed one, which is how and why I found the resources I listed above. Good luck to you.
This is by far the easiest envelope I have ever attempted. I am horrible with directions and was able to follow this one to a T. You can make any size envelope up to a 12x12 paper! Best of all, NO measuring involved!
Uh oh...so I have the martha stewart scoring board...but I don't see an envelope guide. I kept the back piece, for the directions. Was that envelope guide a piece of cardboard? Wonder if that ended up in the trash.
The Martha Stewart scoring board also has a piece of paper in the compartment at the top that holds the scoring tool that has the sizes of envelopes and cards printed on it. There were two with mine which I think are stickers...I'll have to double check.