I was thinking about ways to use cardboard for the garden and it occurred to me that the state liquor stores here in PA will give you free boxes if you ask for them. I'm not sure if the liquor store system exists in other states but it is an idea that I will try this Spring.
If you want to use it for weed-control/sheet mulch, especially of running grass or weeds, the bigger the pieces of cardboard the better.....there are less joints and overlaps (opportunities for runners to sneak through) And you also cover a lot more ground in a given session of mulching. I like to hit furniture and mattress store dumpsters....all those things are shipped in HUGE cardboard boxes. Actually, usually in big plastic bags inside the boxes, so if you have uses for large pieces of plastic (frost covers? greenhouse and coldframe covers?) there's your source for those, too.....
It might not occur to most people, but look up people who do mailings, you wouldn't believe the amount of boxes they have available every day. Usually without any printing either. I do some consulting for a company like this and I know they have to make special arrangements for the waste - can't imagine they wouldn't be thrilled to give it away.
Imagine an unlimited supply of boxes that hold reams of plain paper.
Sustainable Plantations and Agroforestry in Costa Rica