or you can make three little indentations in a spacer yourself. a punch and a hammer might suffice. or a drill...

you might want to check that a four millimeter spacer is enough. if not, the lockring may be bottoming out before reaching the face of the cassette. i say this because i find it hard to understand how what is a essentially a three legged stool can rock, especially since it is being forced into a comparatively soft piece of plastic that should easily deform.