Where to buy a face mask?

I don't think you'll find one without any shape. You can get very plain masks at any craft store, Michael's or Beverly's for example, they are used as a base for making your own mask. Your best bet is to make your own out of paper mache. You can use a balloon as a form. The one on the girl looks like burlap. You can make one like it using burlap and fabric stiffener.