Each of the three campaigns starts you off with your main character as one of the first three (CIA, KGB, or Mossad). The others show up pretty rarely given the freelance bias in the game. Stasi in particular was the most difficult for me to find, showing up after grinding for 30 days at the end of Chapter 4 and another 20 days in Chapter 5. Of course she was also a Beholder sleeper agent...
To speed up this process:
- Start as early as possible. In some of my other runs I found that the agency background of new hire candidates was higher than later on in the campaign.
- Grind for intelligence during a non-advancing stage in the game. Put as many agents as you can on Comms to find more secret documents. Each file you complete should add new potential agents to the hiring pool.