has the Leviton 15 Amp 125V NEMA 5-15P Straight Blade Outlet Rubber Plug (Black, R70-515PR-00R) for $3.98
with free shipping on $25 or with Prime. In stock 8/20. Home Depot
has it for the same price with free in-store pickup. Resists moisture and chemicals, and is suited for outdoor cords.