You could try Geoffry Tailor in Edinburgh as they have their own mills and, unless you want a specific tartan, could probably supply 4-5 yards of double width fabric from their stock on hand. Closer to home you might check with Matt Newsome and see what he (or any of the other vendors on this site) can scare up for you. Your other option would be wholesalers in New York's garment district, but I doubt you would find the weight you are looking for. Either way, good luck!