Unfortunately the last time I used an amount of boxes I just ordered them in a convenient size and slapped some stickers on.....I did find, however, a lot of box suppliers do have an online box design widget which you can use to design your box with.
I'm now not entirely sure what you're after.....surely even if you get hold of a template it still means you have to cut them out once you're done altering it? Or are you going to mess around with the template design and then send that on to a box maker? Or......something else entirely?