Terri,
I use 6 mm clear plastic sheet. Comes in 100ft by 20ft rolls. Check out your local building supply stores. If you don't need that much then you might have to go to a store that will cut pieces to your requirement. Can get expensive.
Professional nursery stores recommend UV blocking plastic but its expensive and my plants grow OK.
Of course I don't have to worry about weight of snow or high winds so you might need something thicker than 6mm.
Would be nice to see pics.
