Building near wood? I love building things, but thus far all i own used is...

I love building things, but thus far all i own used is cardboard inside the house. For Christmas I want to build a kid's clubhouse for my younger brother. But I want to build it outside with wood. 6' * 6' are the round in the order of dimmensions. (length and width) I need to carry wood for this project but I do not have much money to spend...are near any other types of building materials I could use that are cheap and easy to come by? are near places that have approaching second hand wood or something? gratitude
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Answers:    How about pallets as the 4 walls. A tarp or similar matter for the roof. You could cover the pallets with fan-fold insulation, the type explicitly out behind vinyl siding. Lowe's and Depot own it. You could then paint the insulation. Not a impressively permanent structure, but a nice starter project for ya.