Most of your stores will crush their boxes depending on when you go ask for them and/or the baler is broken or no management is around to make a bale. Some stores however may keep a pile stashed somewhere its all a matter of looking and patience! ^^
You can buy cardboard boxes which sounds stupid, but you can if you have a certain time limit.
Just look around and maybe ask them if they can keep larger boxes piled up and pick a time to go pick them up! ^^
Wish you best of luck!