Hi John, if you do want canvas then there are plenty of suppliers on ebay or an upholsters warehousman will have the 16oz stuff. Look up tarpaulin makers in your area - they will have the green canvas for lorry sheets.
Autocamp rooftents use a synthetic fabric called Airtex which you can buy on ebay.de - it's light, strong and waterproof.
DD Tarps do a green lightweight 2x3m and 3x3m ready hemmed and eyed for £20 - £30
Then a couple of strong poles from a good camping shop.
"Never have a stupid argument with an idiot - he gets a lot more practice than you"
there I go again
not too hard really