I actually have a friend who ships her goods constantly and she gave me a stack of cardboard. However when i'm out and about i'll find cardboard or pieces of wood. I either take it home or draw on it right there and leave it for anyone who would want it. Guerilla art yayyyy!
Oh yes I thank her all the time. Sadly she doesn't want a shirt of Carl the Flying Hippo D: Hopefully they will sell well otherwise. Wowo off topic lol. Hey you might find some if you are ever in Philadelphia, New York City, or any town in Westchester County NY state.