This city used to be the outdoor gear mfg center of the world, before everyone outsourced. But there is still a substantial climbing community here, and still a repair shop down the street that caters to expeditions and repairs of packs, down sleeping bags, etc. They keep all kinds of big rolls of Goretex, Cordura, ripstop nylon etc. At one time they even had big a big roll of black Goretex, so I had them make my dream darkcloths for a modest stitching fee with velcro and grommets included. Now they no longer have enough demand for black to invest in a big roll of it, though there are internet sources that will sell it to you by the yard. Otherwise, for more casual cloths, I just let my wife stitch together black and white pieces of thin ripstop. The thought of traditional cloth darkcloths or T-shirts almost makes me break out in hives. I can't think of anything worse, other than maybe cleaning your camera and filmholders with a shedding cat. I have shot below a waterfall with a big Goretex dark cloth, briefly shaken it off, and
it still weighs very little. Try that with cotton.