IF you don't want to make your own, "Scale City Designs" also makes laser cut shims for the Kadees. Theirs are in 4 different thicknesses, English not metric but can get you close.
Also you can just elongate the rear hole in the shim to match the holes on the Kadee box - they are different, slightly closer together than Kadees. No good reason why IMHO.
The Kadee gauge has the a flat on the side opposite the coupler that indicates the height of the underside of the car floor from the rail head.