I used to visit San Antonio for a month every year for 5 years and found that Dibbles is one of the better Hobby Shops in town. There's also a Hobbytown USA that's not bad, depending on what you are looking for.
My first task when hitting any town is to pick up the local Yellow pages and look under hobby shops and start calling. FYI, I just checked the Yahoo Yellow Pages (search term Hobby Shops) for San Antonio and got 25 listings.