I don't really use sheets or flannel that much for quilts but I do buy a lot of fabric scraps and sewing notions at Goodwill. I have found lots of misc. parts and feet for my sewing machines there!
I love to find the novelty sheets for kids quilts, especially the vintage ones. I have used these for fronts and backs. I also love the vintage floral sheets. I have made a few quilts from these, as well.