One thing that I reclaim, and reuse is old art boxes from Sketchbox that get sent to me monthly through a subscription. The boxes themselves are cardboard, with tissue paper and art supplies that are ~$15 in total. Each box is individualized to the themes and works, however they’re about the size of a singular shoe box. 
Now, these boxes are absolutely perfect for rat beds and the tissue paper is absolutely wonderful for comfortable bedding. Thankfully the space in these boxes is usually decently used which allows me to feel a little less guilty about having the boxes to keep in my room. These boxes are usually ripped apart, soiled, and very well loved by my little babies, specifically Miss Brain who keeps her domain in a sketchbox. Sometimes these boxes feel like Amazon boxes and I’m worried that it’s wasteful, but I give them to my rats who happily reuse them and give the boxes a new purpose which includes the adoration of cardboard. Fun fact: when cardboard gets ripped up various times into smaller and smaller pieces, the softer it becomes and feels just like tissue paper and usual bedding that you can buy from the petstore. My other rat Pinky has made this into a fine art, and my third rat Esmerelda is her co-conspirator. Miss Brain has her own box that she uses as her castle of herdom.