You didn't state WHERE in Houston you will be, and Houston is fairly large. So we will assume you will drive any distance. Papa Ben's Train Place on Bellaire Blvd. 100% trains. http://www.papabens.com/ G&G Model Shop on Times Blvd. A large portion is trains. http://www.gandgmodelshop.com/ Both of the above shops are inside 610 Loop, south of US59, and not too far from each other. Up on the north side along I-45 at 1960 is Larry's Hobbies. They have a decent train section. http://www.larryshobbies-houston.com/ Ted Ferkenhoff Flagstaff, formerly Houston
Since Wayne Weiss retired and closed the Houston Roundhouse, I do walthers dot com and caboosehobbies dot com (Denver) For a city its size pick'n's are slim.
Karl makes a point that unfortunately is sad but true. As hobby shop owners either pass away or choose to retire, closing is becoming a growing trend. There are still a few top notch shops around like Caboose Hobbies and Discount Model Trains, but it is not a business for the newby to try and make a living at. Another example of how the Internet has changed our buying habits forever. Just my opinion.