Photography - light proof material (for darkroom, changing bag, focusing cloth,etc)

For a project I researched lightproof material. The absolute cheapest option I found was the "Blackout Drapery Liner" found in most fabric stores. My tests show that is 100% lightproof (though you may want to test for pinholes, just to be safe). The one I tested is Roc-Lon but the one linked to the left is less expensive and just as good.