I would recommend checking with your local grocery store to see when they stock the health and beauty section and if you are allowed to take their empty boxes. That's the best department for smaller light-weight boxes (but make sure you get the ones that are sturdy enough to ship) so that you can still send first class but have the MIB ponies be protected. I usually just go to the grocery store mid-morning and they will have a cart with the flattened empty boxes and I just check with whoever is stocking and take what I need. I just got some yesterday!
Walmart is also an option but most of them stock overnight so you basically have to stalk them after midnight to get boxes.