You can use smoked plexiglass, the ones in the link are a little thick but gives you an idea of what you are looking for once you have the dimensions I used 3mm thick on mine..
Grey Tinted Clear "Smoked" Acrylic Plexiglass 1 4" x 8" x12" Plastic Sheet | eBay ,
Only issue with this is it's flat and will need heat treatment either by really hot water or a lightly going over with a heat gun to bend it into shape, if your helmet is not strong enough you will warp the thing trying to get it in there..
cut it to shape first, heat it then get in position and stick in in place how ever you like, I would get a friend to help you with this or grow another set of hands