It will be available through Amazon within a week, and will slowly filter its way through the rest of the online distribution system.
This volume contains the first half of Season 1. Volume 2 should be available in about a month, with other volumes to follow.
And of course, Shadow Unit in its entirety is available for free online, and as a modestly priced ebook through the usual sources.
The story began in 2007, and will end in 2013. It's not too late to discover one of the coolest collaborative serials in the genre internets!