The HEB chain of grocery stores includes the wonderful Central Market store on Westheimer, and also, two locations of their hispanic themed Mi Tienda stores. My favorite is the original on Spencer Highway but I also love the one off of 59.
This is where you will find nixtamal made fresh from slaked dent corn, on premises. Freshly made torillas that are not made from Maseca.
And a great place to sample different licuados and aguas frescas.