Wandering through Walmart this evening, I found a cheap tarp. Walmart sells it as a sunwall for 10'x10' camping gazebo under their house brand, Ozark Trails.

It is nylon measuring about 9.5'x6' and sells for $15.

They also have a smaller 5'x7' tarp for $10, also one of their house brands.

Conway Yee

This Ozark Trail Gazebo 10' x 10' Sun Wall is designed for outdoor group activities, such as picnics and tailgating. Made of a durable polyester construction, the 10x10 canopy tent wall is quick and easy to set up and take down. This dark gray tent sidewall will protect you from the sun and light rain.
Ozark Trail 10' x 10' Sun Wall, Dark Gray:
Quick set up and take down
Great for outdoor group activities
Dimensions: 10' x 10'
Polyester construction