If they are scratched and such, try toothpaste on a soft rag, buff and then wipe/wash off. Sounds wierd..... but it works! The other option would be one of the commercially available headlight lens restoration kits. I bought one for my car, but first tried it out on my Dad's outdoor thermometer, the plastic lens was hard to see through (and yellowed), but cleared up using that kit.
If the windows are actually cracked or broken, Joe has the right idea, Rudy Nickerson at D&R is your best supplier. (508) 644-3001 or
www.drmarine.com Best to call Rudy, he gets too many e-mails to answer quickly.