Well, the front page of Apprentice Alf's website has a big shiny link screaming "click me"... that has the plugin.
It cannot be distributed through calibre, because then the developer becomes complicit in publishing the DeDRM plugins. Which *can* raise legal issues, as distributing (but not using) DRM-circumvention tools can make one liable for prosecution under the DMCA, which is a headache the developers do not wish to get into. (Nor, for that matter, do the owners of MobileRead.)