I've one missing from my E350, 2011 plate.
TRW (who make them) deny all knowledge that they make them, and Mercedes only sell the whole caliper (£280). Pretty poor from an environmental sustainability standpoint.
As you've deduced by now I'm sure, they are either available from second hand calipers (and might break on removal), or need to be made as you have had done.
Good job getting those sorted btw!