How to find a Orc from a specific tribe in Shadow of War?

First of all, you can interrogate orcs to know the identity of captains or warchiefs, once you have informations on them, you can browse to seek what you look for. Unfortunately, every tribes can be found in every zones, it is random as the class they will have. Don't hesitate to kill the ones that doesn't fit your needs and wait time at the tower so new orcs will replace them.

To make your orc legendary, you can let him kill you a bunch of time but you can also make him fight your followers by ambushing him and letting him win, death threating him etc. Another risky method is to kill them and hope they will "cheat death"