Rear Bumper Screws

I have a question about the rear bumper on my Nissan Quest (1999)
I was hit from the rear, there was no major damage, but my rear
bumper feels loose. Under the rear door, just below the door frame
there are six plastic screws. These are used to screw the bumper into
the frame, well two of them broke off on impact. Where can I fine
them, and how do I replace them..

Thanks