My original mud flaps were a hard plastic, They had become brittle and were breaking apart.
I lost one of them in January some where on I-75 between Dayton and Toledo.
The I visited one of our local Auto /Truck parts stores (Advantage Auto Parts) and bought 4 new rubber flaps for $6 each.
I used the old flap as a guide and cut the new ones to fit, New self drilling sheet metal screws. It took about 60 minutes to redo all 4 flaps..