I've heard of using old mylar balloons, cellophane, empty cereal bags, plastic bags that you get to carry your groceries home from the store, and even the poly/plastic feed bags (anything washable that makes noise). I'm sure others will come along with more ideas. Just make sure whatever you use is washable.
If you search the web for "how to make crinkle toys tutorials" you will get ideas from there as to what others have used in the past.