Most any of the lumber supply or building supply places carry those woven plastic tarps - blue, green, tan, gray. A few years ago, I picked up several tan 12 foot by 20 foot ones from the local Menards store for around $20 usd each. They also have 5x7 feet ones on special sale all the time for around $2 usd. These small ones they advertise for covering your garden tractor or woodpile. Yeah, they are woven nylon strips, and "rattle" like any sheet of plastic will, but you get a lot of very good water repellant cover for very little cost.
Note: these woven plastic/nylon tarps are water resistant - not water proof! They shed water very well, but if you have water puddling on them, it will eventually start to leak through. Some local farmers found this out on their hay stacks when they first came out. Water puddled, then started leaking through, and rotted a lot of hay!
But also go over to the PAINT section, and look at their Painter's Drop Clothes. They have the plastic sheets, but most also have CLOTHE ones. They are generally made of around 10 ounce cotton or cotton/nylon blend canvas. You can generally pick up a 12x15 foot canvas dropcloth for around $20 usd or so. It will not have any waterproofing or other chemical treatment, but that's a lot of tarp for the money - especially when regular canvas is selling by-the-yard for at least $4 to $6 a yard.
If you know a trucker or trucking company, a lot of the big trucks hauling grain have green canvas tarps covering the trailer. These need to be replaced every couple years. The old ones often end up in the dumpster. That's a LOT of good using canvas to salvage! Yeah, it's HEAVY, but they would work great for making a Lavvo or Indian/Sami Conical Lodge!
Just a few humble thoughts to share. Take them as such.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands