Some years ago, I needed to have a new canvas top cover made for a Dingo scout car, this would have been brown canvas in the wartime period. None of the afore mentioned canvas makers were interested as they said it was not possible to obtain Brown canvas. It did not take me long to find a supplier in England and after seeing a sample, went ahead and got a top cover made by a local sail maker. This led to a number of orders from other Dingo owners and all were satisfied with the colour. In fact a forum member asked if I could get a canvas sheet made for his Carrier to the original design and he approved the colour. The supplier to contact is Kayospruce Ltd in Fareham, Hants, Product is called Cotton Canvas, but there are several weights of canvas and I don't recall the exact one I used. Their website shows the colour, but they will send you samples.
regards, Richard