If I look specifically for shipping materials, I simply grab then in stride when moving throughout my city on errand, or in my building's recycling area. I stock up on the night runs as the pics show, but one offs are a breeze for me to find virtually any time.

It's likely a function of my city, but free boxes are virtually everywhere if you keep an eye out for them. I also take other boxes with graphics and cover them in paper/plastic bags and tape to reuse those elements. Thanks for sharing.