I was using the billboard tarps from a local advertising co., but they began selling to the marketer too. They lasted several seasons. Didi not figure out how to make hems and double the thickness so that eyelets would stay. We must cover in the summer too, as we get 64" of rain, largely in the summer. Last year, I got a deal (Woot, which is a clearance co. for Amazon) on black 6' x 20' tarps with eyelets. Since my storage pallets are 4' wide, 30" high, the tarp would cover 10 pallets in a row. Used an idea from some firewood forum of making weights from ~ 3"x12" pieces of limb, adding a hook so that the weights kept the tarps in place.
Due to the continued high humidity, we did have a fair amount of mold/fungi loss.
This year, I bought a case of 6' x 20' silver 12mil tarps for the season.