It will depend on your location and whether you have a ‘brass neck’.(That is, the ability to overcome your normal reticent to scrounge.)
A good source of cheap (ie free!) heavy duty polythene sheet is a farm yard with cows ; if they use/keep silage in pits or ‘sausages'(known as Ag Bags). Normally its a one-use cover, is tough and, usefully, black on one side, white on the other.
See:
So keep your eyes open when you see maize being cut /carted. Its going to a place that will have plenty of scrap plastic after the winter.The real test of a ‘brass necker’ is to get your newfound friendly farmer to cut it nice and strait , fold it squarely, and put it in your car..