recycled cardboard can be found with this tutorial on how to make a realistic Tec DC9 gun. Full details here.
NOTE: As with all tutorials on building prop guns, safety first. Don't be a idiot and go outside waving a fake gun around It may be fake to you, but to others it might not be. Your neighborhood police officer is not a psychic. He or she will not always know if you are making a movie. Always keep that in mind. Know your state laws and get permits or police aid whenever needed when necessary.