It seems the parts are only available from Stouffer Graphic Arts (www.stouffer.net).
I have the Czech ones; they’re available from Phototec:
http://www.phototec.de/advanced_search_result.php?XTCsid=1d0rve57d6hubhdj98mv1ih8f3&XTCsid=1d0rve57d6hubhdj98mv1ih8f3&keywords=graukeil&x=0&y=0
They aren’t calibrated, but they’re still perfectly fine to work with.