Funny you should ask about these, as I literally bought 3 of them at Costco today! To answer your question, I bought them at the Costco in Van Nuys, CA. I'm guessing they would have them at all of the SoCal Costco locations, but you can call to find out.
A few things to note about these tickets. You have to use them for the first time no later than June 30, 2014. There are some block out dates; however, the first time you use the tickets, the block out dates do not apply.
The ones on the Universal website are good from the date you first use them through December 31, 2014. I have no idea if they have block out dates. The Costco ones are good from the date you first use them, and then for an entire year after that. So let's say you first use the Costco tickets on March 31, 2014. You could then use them anytime all the way through March, 2015 (one entire year -- not just 2014). I hope that makes sense.