You have to participate in the online events to get tickets. The only event so far which rewarded a ticket was the Zombie Mayhem event (check the event archives). If you missed out on that, you're going to get another shot with the next upcoming event, Neutralize the Threat, which starts in about 1 day.
WHY SO SERIOUS!? O_o LET'S PUT A SMILE ON THAT FACE!!! >=] PSN: SHIN_MEGIDDO_X http://www.youtube.com/watch?v=lQi0Mv7kK_E