Going between Austin and Houston you will probably drive through Bryan/College Station There is small chain called FX Game Exchange that has lots of retro stuff. There is one in College Station and there might be one in Bastrop which is a little outside of Austin, but I heard it closed. There is a small chain in Austin called Game Over Videogames. There is also a Console Game Exchange in Temple, which isn't too far outside of Austin.
Looking at Game Over Videogames' facebook page, it looks like they have two locations in Austin, one in Round Rock (suburb of Austin), one in San Antonio, one in San Marcos (in-between Austin and San Antonio), and one in Houston.
If you go off course for any reason, there are also FX Game Exchanges in Waco, Brownwood/Early (central Texas), and in McKinney and Plano near DFW. And if you go really off course there is a retro store way down in Corpus Christi.