Joan's will have several qualities of solid fabric. Kona which as been mentioned is one of the best. There are some others that are good as well. Just select a good weave. My suggestion after seeing many many quilts is that when using white, select one just a bit off of pure white. Some companies call it Winter White. It is not off white but just a teeny bit away from pure white. It is my opinion that this blends better with other colors. Bleached muslin for me is too white and the unbleached is too dark. In quilt shows I have seen many quilts of older patterns in which the pure white was used and they lacked character because of the starkness of the white. It is a white probably toward the cream rather than beige.