You can get KOS troops from prisoners you free and by other means, but it's all pretty rare.

The best consistent way to have KOS troops is to own a castle and have the KOS order in that castle, from then you can do quests for them or recruit them by upgrading troops, but it takes some ressources.

In a very long playthough, your own KOS order troops will likely be more powerful than the ones you find in the wild.