The information about Alamo is listed on Mousesavers.com. It includes the phone number to Disney Preferred Ticketing, which supposedly is an official Disney company that sells these tickets.
I would strongly advice getting the tickets far enough in advance to have the tickets sent by mail. I purchased the 3 day park hopper plus pass about 1 month before we left. When I did not receive the tickets in the mail after the two weeks they said it would take, I called back and they said the tickets would be at will call. When we arrived at will call, it appeared they had given our tickets to somebody else. It took over 1/2 hour with the will call CM on the phone with the office and me on my cell phone to the office. Disney gave us 3 priority passes to any attraction where we could go to the front of the line, which was nice. By the way, my sister in law picked up her tickets from will call right before me, and she didn't even have her confirmation number with her. I'm not sure if they gave her my tickets, or they gave mine to someone else, but it was a frustrating 30 minutes at park opening on our first day.