Missing plastic on bumper, need help!

My right side of the bumper has been sagging since i bought it, i finally got a chance to take a look, and the plastic that fits into the end of the m3 bumper, the one that slides into the bumper bracket thats mounted to the car, is missing on that side, so there is nothing holding up the bumper there.. i searched all over the net and no luck finding it, can someone at least tell me what its called, maybe ill try calling the dealership. Thank you.

If pictures are needed let me know i will provide some

I drive a 00 328ci with a front M3 conversion.

Well here is a picture of it.. will someone tell me if its possible to buy this plastic without having to buy the whole bumper ?