Wheat can't be found in the wilderness. You can find crops as loot in several containers and you can find ready grown wheat in some POIs, like the Farm or the Supply Station. But even with these places wheat is still a rare ressource. Fortunately it's not really important. You only need it once when doing all the PDA missions (for a cake and a cookie). After that you never need wheat again.
If you still want to have some crops for some kind of backup you can make your own crops in the food processor, given that you havn't converted all your wheat into flour ;-)