I know this is a really old post. However I figured that it would be a good idea to answer this question so that anyone who does a search (like I did) in Houston area could find answers.
For starters if you're in a crunch most Petcos sells a very expensive bit of flies that will probably mostly last one or two feeding tops. The only actual reptile store I know of that carries them on a regular basis is Rainforest Pets. They are in the League City area about 20 minutes south (down I 45) of the city center.