From their site:
Dreadfleet will be disappearing from the Games Workshop website at midnight (PST) tonight. So, if you haven't grabbed your copy yet, make sure you buy one before they sail off into the Galleon's Graveyard.
Some Games Workshop Hobby Centre's may still have a few copies left in stock, but you'll have to be quick to get your hands on them.
So move fast and get your copy before they sail off into the sunset.