There's only one MX rear fender available for the KDX. Its made by Maier and comes in green or black. Most places could get one for you including KDX specialty shops like FRP of Sage's dirt and street.
Most people will modify a late model KX numberplate to fit their KDX. A YZ number plate can also be made to fit as will a later model RM number plate with the integrated fork guards. All will take a little work but nothing the average Joe couldn't handle.
David