There is no choice but to fly to Houston for a Galveston sailing. There is no airport in Galveston. There are however two Houston airports, IAH (Bush Intercontinental) and HOU (Houston Hobby). IAH is the main international airport and the only one the cruiseline air sources use. Continental Airlines is based there and will probably be the most attractive pricing from most airports into that one. IAH is approx 70 miles from the pier and if you're flying in on the day of the sailing (I'd come in the day before) the cruiselines offer transfers via motor coach both directions. If you're flting into Hobby they do not offer transportation even though it's closer to the pier (about 54 miles). Some of the discount airlines fly into Hobby, like Southwest and Jet Blue. You can rent a car at the airport or online and drop it off near the pier and the rental company will provide a complimentary shuttle from their drop location to the pier. The other option is an outside transfer company. The one I have recommended to many clients without any issues is www.galvestonlimousineservice.com
They provide transportation from either airport and are normally a little lower cost than the cruiseline transportation. Hope this helps.