I still get the product Dan is mentioning here at the Home Hardware where I live south of Ottawa? It lists as a wood preservative but can be used on other materials such as canvas, buy the clear version. It now has only 3% zinc naphtenate in it down from the 5% previously but I've had no problems so far.
I spray it on, outside, not on a windy day, and when I know I'll have at least 2-3 days of clear sunny weather. It's horrible stuff to get the smell out of so wear old clothes, and rubber gloves.
Or as Dan suggests buy treated canvas!
Gary