Well there are different types of plastic you can use. If you are doing an ROTJ then you want black, if you are doing ESB you need green. But you can get the black ACRYLIC. Its a clear plastic. Its typically called smoke tint though. Find a plastic supplier or Sign Fabricator in your area and ask them for smoke acrylic.
You can buy pre-cut green T-visors at http://www.t-visor.com/, as well as on ebay. however, both these only carry the green tinted plastic. I know it may seem suprising, but the T visors you see on most helmets is not black-tinted, but green. this may be for a few resons. green plastic is easier to see through, as it only tinges everything green. whereas black or smoked plastic has more of a sunglasses effect, which, in my opinion, is slightly more dificult to see through.
if you want a totally screen acurate ROTJ T-visor, Gilmore of OK is right, dark or smoked plastic would be best. However, dark green visors often appear black since the interior of the helmet is rather dark, and incoming light is somewhat reflected. in short a green visor could easily pass for a dark one, depending on what you prefer and what's available.
good luck
I picked up a dark green tinted face shield from my local Ace Hardware / True Value for use in my Phase II Episode III Clone helmet.
It's only about $6 and I can get 3 visors out of it. Of course with the longer Mando visor, you may only be able to cut 1 or 2. You can even cut it with ordinary scissors.
I think this is the part number:
Forney Industries 58603
You can buy pre-cut green T-visors at http://www.t-visor.com/, as well as on ebay. however, both these only carry the green tinted plastic. I know it may seem suprising, but the T visors you see on most helmets is not black-tinted, but green.
when did that happen? i've gotten at least half a dozen visors from him and they've all been smoked. the one in my current helmet is from t-visor.com and is definitely not green.
I used a Jackson Category 5 welding shield on my Ep III clone. They're a very dark green, but once they are installed they look jet black. Easy to get at any welding supply place for about $15.