There are many online retailers that specialize in aftermarket grilles, such as Stillen, American Truck N' Auto Accessories and T-Rex Grilles. Many offer several grille styles for a limited selection of trucks and sports and muscle cars, although some can custom-fit a grille to your specific vehicle.
Many companies which sell aftermarket grilles carry many other products for vehicle customization, such as air intakes and trim pieces. Some sites offer free shipping on some or all of their products.
An aftermarket grille is also called a billet grille. Most are designed to fit directly over the existing grille for easy installation.