You can find those at any local hardware or auto shop. It doesn't really matter what grommet you use, as long as you find one that fits snugly into the door. I recently removed the piece that fits along the underside of the car up against the front bumper, which is attached using several similar plastic grommets (and 4 metal clips on the front side of the wheels). I found that one of them was missing, though surprisingly the screw was still there, so I popped in another one before putting the piece back in place - it looked nothing like the rest of them, but it works fine. So don't worry about it's appearance, just get one that fits well so it can do it's job. Also, I don't know why you couldn't just remove them from the old door. Usually, there are two tabs on the inserted side which you have to depress in order to allow it to back out of the hole.
