It's hard to give better suggestions without knowing the helmet this would be for.
Depending on the helmet and the visor shape that you're wanting you could buy a flexible visor material such as the ones sold by
Icon Props or buying something like a helmet visor (think from motorcycle helmets), face shield, etc. The Icon Props sheets already have a color and will reflect back. The others can be dyed and chromed to make the one way effect just like vacuum forming your own.