Wheat

Hay Bales

1 hay bale can be crafted into 9 wheat. Hay bales generate as piles in Plains villages, and also in Desert and Savanna villages to a lesser extent. These piles can have up to 25 blocks in a single feature and can be found additional to the hay bales which generate in some houses and farms (e.g. Desert shepherd house and Plains butcher shop). 

In Taiga villages, hay bales can't generate as piles, but underneath these campfires (see below) there will always be a hay bale. Neither of the chimney campfires has a haybale underneath. 

In Snowy Villages, hay bales can only possibly generate as part of horses_4 in animal pens. According to Sam's work, these generate in 2.5% of Snowy villages, and they're a 3.8% chance in small animal pens and a 7.46% chance from the larger ones. 

Pillager Outposts can generate with features on 3 of the 4 corners around the tower, and one of the 7 options includes 2 hay bales (see below). Each feature is weighted evenly (1/13) against a 6/13 chance for an empty feature, however since targets take up less space they're relatively more common. Even if you don't get Targets, the outpost chest is very likely to have wheat (see Loot below).

The wheat is then replaced randomly depending on the biome, meaning that for wheat to be in farms, all of the other possible crops failed to roll. This means that the chance for any given farmland to remain as wheat seeds is:

PLAINS: 50.4%

DESERT: 72%

SAVANNA: 90%

SNOWY: 18% -- These will not grow, so will remain as wheat seeds

TAIGA: 56%

Wheat seeds can be grown into wheat, but natural growth is generally too slow to be viable in Draftout, meaning you will want bones/bone meal to grow. On average, you need 2.39 bone meal per wheat (with a minimum of 2 and maximum 4 without natural growth/fortune). Offhand the seeds, till the dirt with a hoe and then hold right click with your bone meal, left clicking when it's fully grown. 

The game won't let you plant seeds in farmland if it's too dark. The easiest ways to get around this in caves (without torches) are using flint and steel, or overlapping smelting with any lit furnace. Alternatively, lava and standing on redstone ore also works, but redstone torches and glow lichen are not bright enough.

Loot

See the screenshot below from the wiki regarding wheat in various loot containers.