As a complettist, I play with an extra laptop next to me, and while many websites exists that list the missions available by planet etc, none seems to provide the exact location of NPCs, just the name. So that leaves you to look around sometimes. I think one of the issues is that there are some NPCs that move around as your quest line progresses. That makes tracking them a bit harder.
There is a great article on this forums about the list of heroics and bonus by planet, and it lists where to pick them up. It helps quite a bit. Link follows :