Sad, but if you Google 'bumper spacer shims' you end up back at this post, so obviously not a common item 
My guess is they are just something someone found to make a bit of adjustment. Can't see why theyu would as you can move the bumper cover in and out on slots and raise/lower the bonnet closed position.
My guess is they are just something someone found to make a bit of adjustment. Can't see why theyu would as you can move the bumper cover in and out on slots and raise/lower the bonnet closed position.