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for the tauren, you talk to the warrior trainer in bloodhoof village and he sends you to uzzek, south west of the the defensive camp on the border of durotar/barrens.
for the orcs/trolls, i would imagine you talk to the warrior trainer in razor hill, who will then most likely send you to the same place.
i believe berserk stance is given by the trainers in the capital cities, TB and org.
cant say for undead wars, but i would imagine its something similar.
actually any trainer can give you the quest, I only noticed that last night when i started an alt gnome mage. When my little mage hit level 10 the mage trainer in Kharanos had a quest for me, but i wanted to turn in a quest in IF first. After i found the guy in IF and turned in the quest, i sought out my trainer for the level 10 spells. Lo and behold he had the quest for me too!
So i believe that any warrior trainer will give it to you (in the case of main cities lik Ogrimmar then ONE of the trainers will give it) as long as you meet the required level :)