canvas is expensive stuff and its dificult to find "off the shelf" in heavy grades and poles made to measure probably not worth the effort .
probably your best bet would be to buy an old frame tent and use the poles to fabricate a frame and cut the canvas to fit over it ,sewing it all together would need professional machines your bog standard sewing machine couldn't cope with canvas.
if you have plenty of money anything could be made but on a shoe string within reason you can make anything as well! only the finish will show the difference
You can buy frame tents at boot fairs we bought our relum for £20 but will now we bought our TT will wait in its steel box in the loft (steel to keep those naughty mice out) incase we need to find a bit of canvas for repairs.
i am terrible at not getting rid of things 3 tents in the loft and one in the garden ,ZIPS! we got plenty off all lenghs and curves "just in case" may set up a tent zip shop
we sent for some samples from there but the canvas is very thin ,not ten carte or near ,fine for internal type applications but not for tent fabric on a TT ,the widths also are limited
these sell wider but i haven't a clue to its quality
http://www.sailcloth.co.uk/