Do you mean the black plastic trim right at the bottom? You can order that from your BMW parts centre. Have a look on
RealOEM.com Online BMW Parts Catalog and locate the part number then give them a call.
If you have the aero kit fitted it is all one piece though, so might be more difficult depending how bad it is.
Many people get it all colour coded, it gives a nice finish, especially in the darker colours, check out some of the sponsors of this site at the bottom of the forum list, I find them quite helpful.