We build our Halloween house out of recyclable materials, most of which is repurposed cardboard. Over the years we have found many sources of large cardboard (cabinet builders, door manufacturers, truck insert retailers, appliance dealers), but one of the most dependable sources we have encountered is bike stores. Often these stored generate several boxes a day as they build new bikes. Faced with the opportunity to give away their boxes without having to break them down, they are often very happy to allow cardboard harvesters like me full access to their pile of bike boxes.
One long-time helper in this regard is Performance Bicycle of Mountain View, CA. This year, one trip to their store yielded a mini-van load of cardboard which would serve to cover the remaining two-thirds of the house. Thanks so much, Performance Bicycle!