When my oldest was "baking", I was doing some painted embroidered squares. Got them done and my mom sewed them together for a top. Didn't have anything for the batting, so mom had an old blanket and we used that. We used a sheet for the backing.
I've since have gone in a pinch and bought cheap blankets for batting in quilts that I know will be used for car trips and for those that will just trash the quilt within a few months. Would get the blankets at thrift stores for just a few dollars.
Oh, and my daughters that I made? It's still going strong 28 years later......