Costco is clearing out their Heavy Duty Tarps for $18.79 - item #467428
You get a 2-pack of 12 ft x 16 ft

Features:
- 14 x 14 Weave for Superior Strength
- Heavy duty fabric Weight 5.5 oz/yd^2
- Two colors for Versatility
- Plastic Corner Bars
- Grommets spaced every 2ft.
- U.V.I. Coating
- For Extended Outdoor use

I originally bought a tarp from Princess Auto (15x20' - on sale for $26.99) and then returned it when I found these.