I often purchase whole bolts from JoAnn's of a white on white. I have one that is a favorite that is a really small floral, but I have used stars, dots, and small scrolls. Of course, I watch for coupons and it is just a thrill to dip into my bin of whites/creams and find all I need to start or finish a project. From time to time I will purchase JA, highest thread count white muslin.
Thank you everyone for the ideas of where you are finding good backings are a bargain price.