I haven't played that much the only special troops I have encountered are Artificers (any country with the Artificers estate), Mythical Cavalry (Jaddari/Jadd Empire) and Ahati Infantry (Available to Steward of Sorrow though Kethist religion is required for the Steward of Sorrow reform giving them +5 discipline and +20% morale recovery).

So currently in the game I am playing Jadd Empire and I have both Mythical Cavalry and Ahati Infantry (I expanded towards the Steward of Sorrow and shared a border with them and claimed it from them)

Elves are also kind of about having elite units as they have -50% manpower recovery but +10% discipline and +100% drill gain and -25% drill loss (and +15% movement speed). You can even get to -100% drill loss with -50% from 100% army professionalism and -25% from the tier 5 government reform (though that would also mean not picking the reform giving +20% special units force limit). -100% drill loss does include reinforcement as well so if the unit is at 100 drill it will remain that way until it is destroyed/disbanded/consolidated