Finding troopers is easy, finding "good", "reliable" troopers is a work of patience.
The game will provide you with 3 free "bot" troopers that will work for you, but you will need to find the others as you progress in the game.
When you reach top level (100) on your profile, you'll unlock the last trooper slots and have access to a whopping 50 troopers (and the 3 bots). Never enough, sometimes too many.
You can find random troopers in the game's trooper screen.
You can also use one of the game's communties to go and ask for troopers. To do that, remember three important things :
It is important to note that the trooper search screen also allows you to look into the roser of anyone whose ingame name you know, handy to look up equipment or toon setups.
Your current trooper list displays how long since the player last played, allowing you to get rid of inactive players.
A large number of players have a habit of "cleaning up" their trooper rosters for troopers who play infrequently, so don't feel bad if you got kicked during your vacations. Giving a warning in your favorite places (forum, facebook group, discord) can prevent that.