I've been working with acetate this weekend because a piece in progress needs some transparent colored wheels as embellishments. I finally found acetate yesterday at, of all places, Tap Plastics! It comes on large rolls and in multiple colors, and the clear comes in multiple thicknesses.
Interestingly...I read an article this morning in the latest Quilting Arts magazine about Katie Pasquini Masopaust and her work. In it, she describes working with acetate as a standard part of her quilt design process!
Acetate is fairly easy to work with including sewing through it. It does have a desire to curl, so to use it flat requires some manipulation. In any case, it was great to have found and just perfect for what it is being used for!