The stuff you want is polaroid circular polarizer sheet. You can get it from Edmond Scientific.

They sell it in a 3" x 4" sheet for 14.50 in the 1999 catalog but should still be close.

They have four types

10% Neutral P/N J53-332
37% Neutral P/N J53-335
Amber P/N J53-338
Red P/N J53-341

Those part numbers are from catalog N997A and their phone number is (856)573-6250

The 10% only allows 10% of the light through and 37% lets through 37% but both will block about 98% of the ambient light. The catalog doesn't say how transmissive the red and amber are.

-Wishing my Mark2 hadn't got stuck in customs.