You can buy clear plastic sheet in the craft department at Wal-mart and you can cut your own from that. My set of plans is at the house and when I get home I can scan the cutting pattern for you. At least I think the pattern is on the plans (so don't hold my feet to the fire). Even if push comes to shove you don't have to buy one, you can cut a replacement from a soda bottle. I've cut plenty of windshields from soda bottles in the past.

Ken