Here's one of the sites that sells them - http://www.craftkaboom.com. Yesterday I made my own with 2 fabric rotary cutting mats (Purchased at Joanne Fabrics store). The mats have rulers running horizontally and vertically, which makes measuring really easy and accurate. All I did was push the mats together, leaving an 1/8th inch gap between them which serves as a "crease maker". I then nailed small brads around the mats to keep them in place. After measuring out the dimensions of the box, I placed the card stock in the appropriate position over the mats to make the creases. I ran a bone folder (a creasing and folding tool made out of bone, also purchased at the fabric store) over the cardstock between the 1/8 in gap to create perfect creases. It worked very well.
Again, thanks for your suggestion of making them on my own. That seems to be the way to go. Have you ever tried using a heavier paper stock, like poster board? It's probably expensive? Also, it may not go through a printer.