Tickets are on sale on from the official BlizzCon site.
If you don't get tickets today, there's two more chances to do so. This Saturday, April 27, a second round of ticket sales will begin at 10am Pacific, and when that's done, all of the regular admission tickets will be gone. Your last chance, then, will be on May 1 at 7pm Pacific, when Blizzard will sell 200 tickets at $500 a piece for the Blizzcon Benefit Dinner, which includes full admission to the show and a charity event.
Update: 29 minutes later, the first round of tickets is sold out. If you didn't get yours, you'll have to try again Saturday.