I'm not sure of whether the shade of tan matches your need but these people advertise #8 or 18 oz. canvas in tan as well as OD.
http://www.bigduckcanvas.com/number-...anvas-duck-tan
They sell as heavy as #1 or 30 oz. but anything heavier than 18 oz. is "natural" so you would have to find a way to get it dyed.