Polycro plastic--the kind sold for shrink-to-fit storm windows and doors--is more durable than plastic painters' drop cloths, and about the same weight. Gossamer Gear and Mountain Laurel Designs both sell polycro ground sheets, but because of shipping cost you can probably get polycro cheaper at your local big box hardware store, unless you're already ordering something from the above two firms.

I don't use a ground sheet under my tent, and haven't missed it, either. I would use one if I were going to camp on snow, though.

Frenchie, I've also heard that kite Tyvek is lighter and more waterproof than the house wrap kind.