Two can be found inside the Nipton house just opposite the Nipton General Store where you find Boxcars. They are on top of the oven in the kitchen in a pot.
Two or three maize can be found at the raided farmstead where the Vipers have set up camp.
Two maize plants can be found at Wolfhorn Ranch, just down the hill from the farmhouse.
Maize plants have appeared in previous Fallout titles, and have been described as mutated versions of pre-War maize.
Maize is one of the most efficient (in terms of Starvation/Weight ratio) Hardcore mode food items in the game should the Courier have low Survival skill, as it often removes more Starvation/Dehydration than most food items, contains zero radiation, is extremely common, and only weighs a quarter of a pound.
Edit