I made my own..
Polythene sheeting from the local garden supplies place (plastic greenhouse sheeting), and a roll of 'all weather' tape. (again, designed to stick plastic greenhouses together)
I cut a piece of plastic long and wide enough for the sail, folded it in half width-wise, and taped two of the edges (side and bottom), turned it inside out, and taped the same seams from the other side.
Total spend for three bags, and most of a roll of tape left over (which is brilliant for taping sails) $33.
Two seasons in, and the bags are still going strong - just like the car.
I can't imagine why there's not a commercial product around - it's not like it'd be a complex thing to manufacture.. Welded vinyl sounds like the plan for a commercial product.
Something like this, only long enough for a sail: