Rare units are units randomly dropped by enemies. There are three categories: Rare, semi-rare, and ultra-rare, according to the chance of obtaining them. Rare units are rather common, semi-rare units are slightly harder to find, and ultra-rare units are extremely difficult to obtain.
Buying the Zava Detector upgrade for the Tech Center will increase your chance of getting rare units. Whether it stacks or not is being debated. In addition to the Zava Detector, bonus content increases the chance of finding rare units, especially ultra-rare.