The only quilt shop that I know of near San Marcos is B & B Quilting in Buda, just north on the freeway towards Austin. I have never been, but have received fabrics from the shop. Here is web site info:
http://www.bbquilting.com/ Hope you find some good fabric on your trip. There is a Hobby Lobby in San Marcos that has relatively nice fabric. The Walmart there only has precuts unless they have changed over in the last month. My daughter lives there so I go every week.