You need to research to get farms. Without the game open in front of me, I couldn't tell you what, though. Try checking the planetary improvements chain - the one leading to terraforming, or possibly extreme colonization.

Yes, except some civilizations, like Yor and Iconians don't get the best farms in the game, just some crappy ones, so they should buy farming from other civilizations.

Yor farms are awesome.

While disabling farms can be useful in an economic or influence battle, you are usually better off using spies on morale structures and using Information Warfare when invading. Not to mention the fact that the same spy can be used to attack multiple planets per turn, and that the spy does not need to be left on the planet for a turn to be effective. Damage from a morale spy is instant.