Here is a map that I created listing most of the international / ethnic grocery stores or markets in Houston. The multicultural aspect of Houston led to places catering to people from around the world. Some of my favorites are: Acacia for Turkish products, Las Delicias, El Bolillo, Phoenicia, Super Vanak, Subralaxmi, Mi Tienda, and Nippan Daido.
Here is a screenshot of the map:
And here is a link to the actual map:
Ethnic Markets in Houston