It's available on pre-order from:
The Book Depository (includes free shipping worldwide)
(At time of publishing, The Book Depository price was cheaper than Amazon - which may change over time.)
There's no news yet if the calendar will be included with any upcoming fan club renewals packages.
**Edit - Josh has responded to a tweet from a fan about the calendar and advised that it isn't official. It has been subsequently been removed from the US amazon site. Many members how had ordered it through this site have been emailed that their orders have been cancelled. However, it still appears to be available on the Amazon UK site as well as the Book Depository.
@fallingleaves4 it wasn't a proper calendar, haven't done new pics yet!— josh groban (@joshgroban) September 21, 2012