El Rancho, Fiesta, Terry's, or El Mariachi are all the supermarkets around that cater to the Hispanic populations. I am not sure what some of the Carnival stores turned into. Like DD said you might could find it at DFM. I know McKinney, Lewisville, Grapevine, Coppell and the smaller farmers markets do not have any hispanic produce. Exception being jalapenos when they were in season.
If you have a constant need and I am not trying to be snarky about this but next year I would get several plants and grow it in a planter (if not in the ground). Harvest it regularly and pack the herb into ice trays, put in some water, and freeze it into cubes. Thus would allow you the chance to use it in the "off" season. You could also buy extra when you find it and freeze it.