Awesome idea, Deborah! It sounds like a marketplace concept where you are connecting people who have materials with people who can make use of it.
Once it reaches scale, it would be great for the recycling movement and bring more awareness to more ways to use material that some people would consider as landfill. While in the process of scaling, I image that it is more difficult to get people to actively contribute the materials than find people who want them. In addition, the people who make use of the material might often sell it to someone else who needs the end product.
Have you explored closing the loop so that the person providing the material would also be the consumer who wants the end product? In this way, you can (a) solve the problem of incentivizing people to provide materials and (b) solve the demand problem for the person making use of the material.
You can easily test this model by contacting a few makers (maybe an Etsy store owner?) to see if they are willing to charge a lower price for the product when they are provided with the material. I know for one, I have lots of wine bottles that I am hoping someone can help me make a chandelier out of. As an added bonus, I can imagine my friends who see this end product to also want it. In this way, the word can spread and habits can form.
I would love to help in any way I can! Just let me know how.