FFG site, they have the expansion rules posted in an article about how the new box set is forthcoming. In my experience, releasing the expansion rules via PDF is the last step before the sets hit retailers. At least when dealing with FFG anyway.
This is pretty exciting, as a lot of cool new features seem to be making their debut in this expansion, including fliers. Also, those Battle Commisars look like just the ticket to boss around my 'Recon Boys counts as Everything' squads from a while back.