X-ray film is usually exposed and then recycled at hospitals after it is used. Go to the dept and ask them to keep some for you, explaining what use you have for it. What I have received has been free. Since you are wanting something you can see through, try using a flat piece cut from the side of a milk jug. My mother used to save the plastic "tray" that is in most brands of bacon, wash them well to remove the grease.