They also have tin cans, plastics, cloth, etc.
Parents can be a good source of materials.
Many parents in our area work in technical jobs. Their companies may have excess supplies that they need to get rid of. For example, one of my student's mother works for Bayer. Her company gave our school 1000 plastic test tubes and holders because they had received the wrong size tubes from their supplier. For those in the Raleigh area, a good place to pick up cheap materials is the flea market at the state fairgrounds.