If you reach a ceratin level of the tavern, with countermeasures, spies will be unlocked as well. So if you open the tavern, you can see the available researches, and on the indicated level, spies will be available as well.
For more informations about officers read this wiki article:
I see when countermeasures are unloaded but I don't see anything about spies. Usually there's a lock with the level that unlocks it but there is none for spies.
And as to the wiki, it doesn't say how you can get more.
What I've noticed is as you unlock countermeasures you unlock a spy. They should have the lock symbol on each spy telling at which level it is unlocked just like units and buildings.