I used cut up small pieces quilt batting because I am a quilter and have those scraps to hand. Other ideas to consider would be: cushion stuffing from old cushions you don't want; cut up scraps of fleece (either from sewing projects or old clothes); scraps of soft sweatshirt or t-shirt material. The key is to pick something soft and something that can be cut up small enough so that it doesn't bunch and create lumps in your bunny's body.