Looking for the air dam the plastic piece under the radiator does anyone know where I can buy this part it's for a 70 cutlass

 

I'm not aware of reproductions, but this part came on every single A-body Olds ever made, so there are many, many good used ones out there. Technically is an air deflector to direct air into the radiator, as opposed to an "air dam" that usually means an aerodynamic aid to improve handling or reduce drag. The plastic part really just closes out the gap between the bumper and the core support so air through the grille goes through the radiator and not out the bottom of the car under the radiator.