Amazon has the Performance Tool 4.5' x 6.5' Tarp (Blue, 4mil Thick, ‎W6000) on sale for $4.45 with free shipping on $25+ or with Prime. Aluminium grommets every 3 feet, reinforced corners. 

Editor's note: Amazon lists this as 5' x 7' and 10mil thickness, but manufacturer product image indicates 4.5' x 6.5' and 4mil thick.