Ticketmaster online allows you to order tickets on the web.. you must incur their service charges... http://www.ticketmaster.com
You can also get tickets from local ticket scalpers/ticket services. There are alot of those guys in Houston. I don't know the names of any off hand, but I'm sure if you did a search for Houston Tickets or Tickets, Houston or something to that effect you could find these guys on the 'net.
I would rather incur $5-6 service charge than $10 or $20 more at a "ticket outlet/scalper"
You can also get a ticket at the box office on the day of the game, probably. There are very few games this season that are sold out and your chances of getting a single ticket are really good. It's harder to find 2 seats together, unless you are willing to spend 25+ bucks a ticket.
I hope you get a chance to see a game, while you're down here.