I use sheets in my projects as well. If you prefer a higher quality fabric than muslin, you can certainly purchase with coupons regularly.
there is quite a difference between flannels and quilter's cottons....there are quilter's flannels as well. It can be very confusing, especially when you can't feel the difference as in ordering online. Quilter's Flannel is 100% cotton and it has a much better wear than cotton/poly blend flannels also known as flannellette but sold in chains as flannel. People have debated whether you can combine these in with traditional cottons in quilts, but you should know that it is slightly harder to work with and has a different texture other cottons.
Good luck with your projects & happy sewing!