If you have a local to you sign company that does billboards, they will have them for sale. I use them all the time for covering stuff. I use a grommet setter and brass grommets (Tractor Supply) for hold down points. Most are neoprene coated nylon fabric web, very strong. Easy to cut with shears and easy to deal with and last 10 times as long as the cheapo blue plastic tarps that fall apart....
I bought some that had the 'Feel the Velvet gal on them and used them to cover my barn roof while I was working on it. Got a lot of people driving by looking at the 'Feel the Velvet' Gal.....lol
Typically, I can buy a 20 x 40 for about 10 bucks locally.