Welcome to the Lone Star State! I've lived in Austin for the past 12 years (but I work in California...does that make sense>) and don't wish to live anywhere else. When I lived in NYC, the last place I ever imagined myself living was Texas...but now I can't imagine living anywhere else.
To answer your question, I've been buying my balloons from Balloon Ideas in San Antonio. They have a good selection (Tuf-Tex) and excellent prices. However, I'm not sure if they have a storefront or just an online presence. They list a physical address but not sure if it's a retail store... http://www.balloonideas.com/index.htm
There is also a good store, from what I hear up in the DFW Metroplex but can't remember the name.