A good cheap source of canvas to use is as near as the nearest trucking company. Most of the big trucks that haul grain have a canvas cover for the trailer. That canvas cover needs to be replaced every so often - due to age, tears, rot, etc. Well, most of it just gets tossed into the dumpster.
So, if you know a trucker, or a trucking company that hauls grain, go ask them about ... salvaging ... those old canvas trailer covers. They are HEAVY, waterproof, and often are green in color. One cover from a large trailer should be enough canvas to cover a LARGE lavu. Just cut it into the lengths you need - or can "carry" easily.
You could always use those woven plastic/nylon tarps from the lumber yard or building supply store, but real canvas just looks and feels better. And it doesn't "rattle" in the wind!
Hope this helps.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands