As one of the "triad" of foodstuffs on which America's great indigenous cultures was based, you can bet that a wonderful variety of corn (maize) grains can be found in mercados. Often corn is shipped to mercados in bags such as those shown below.
The most commonly sold corn grain is the pozolero type shown at the top in the photo at the right. Pozolero corn grains are large, white, and relatively soft -- perfect for soaking, grinding, and making into tortillas.
Often pozolero corn grains are sold with the germ removed. This helps the grain to store better, but a less nutritious tortilla results. In the photo, notice how the corn grains at the top left have had their bottom parts chopped off, thus removing the germ. All the different varieties of corn derive from a single species Zea mays of the grass family. Corn originated in Mexico.
Other grains you might spot in a good mercado include alpiste, or birdseed, shown at the photo's bottom, left. In real life, alpiste is tan colored. In the picture, the grains at the bottom right are arroz, which you'll probably recognize as plain old rice, Oryza sativa.