Trolls will spawn near copper deposits and troll caves you can also go inside the troll cave and usually find a troll. Even after you have killed the troll inside the troll cave another can spawn outside the troll cave 3 game days later but no trolls will move back into a cleared troll cave.

You can also kill a troll and the elder and just hang out inside your base and wait for troll raids. 2 will spawn per raid and if you go outside the raid circle to kill them the raid timer pauses. When you've killed those 2 trolls you can go back inside the raid circle and 2 more will spawn. Rinse and repeat until the raid timer expires.