In the Houston area, there are 3 that I'm aware of;
1. Papa Bens in the Meyerland area.
2. G&G Model Shop, in the West University area.
3. Trains, in the Spring area.
I've only been to Papa Bens, he has a good selection and is helpful. The prices are okay. I've not been to G&G. I've heard about Trains from a friend. His review after visiting the place was like many online reviews, the place appears to be quite disorganized, but people keep coming back.
Back in the 80s/90s I used to go to JR's Hobby, but it is gone.