what is the hard plastic under radiator and the screws that hold in in place called on a 2007 Pontiac grand prix

Without seeing a picture of the part in question, my best guess is the splash shield/air dam that attaches underneath the car. You can get most types of body fasteners at your local auto parts store. If you need to replace the splash shield try a local pick-a-part or salvage yard.

If it is the air dam, you should replace it to make sure your car gets enough airflow past the engine. Without it you may experience overheating problems.