I've just come across your query, so couldn't reply earlier, but found the replies helpful as I have some decks to re-canvas soon (when I've painted the interior and laid the decks). My sailmaker put me on to Whaleys (Bradford) Ltd, Harris Court, Great Horton, Bradford, West Yorkshire BD7 4EQ (01274) 576718, email@example.com, www.whaleys-bradford.ltd.uk. I phoned them and they sent me their catalogue and price list and samples of their loomstate cotton canvas (which comes in widths of up to 9ft).
But I still don't know what weight (thickness) to choose!!
Many thanks for this advice. I got the samples and the prices are excellent. Because I am covering a cabin top I do not have the need for canvas to withstand foot traffic hence I chose a 9 ounce material.