Another update: I contacted a couple of "narrow fabric mills" in the US. They can make 18mm width cotton webbing as thick or thin as I want, in any weave and colour. But . . . "we would have to produce perhaps 7,000 to 8,000 yards." (7000 yards is about 4 miles.)
Next step: cutting up some WW2 bags . . . . I'll keep you posted.
Incidentally, my wife thinks I've finally lost the plot and suggested I should be making straps for a straightjacket. I might yet.