In most of Mexico's Indian lands, even in villages where the predominant language is native American, not Spanish, most people wear regular street clothing. In more conservative areas, often the men dress in street clothing, but the women attire themselves in traditional dress. Only in really isolated areas, usually deep in the mountains and especially in Chiapas, do both men and women dress in the old ways.
Though in mercados we often find stalls selling serapes and perhaps even some pretty, embroidered huipiles, conservative Indians tend to acquire their traditional dress outside mercados. Perhaps they buy from or barter with someone in their village who specializes in making the product, or maybe they or someone in their immediate families make their own.
If names like "serape" and "huipil" through you for a loop, you might be interested in the following list of useful Mexican-Spanish words:
You may be interested in reviewing Textile Traditions of Mesoamerica and the Andes : An Anthology