From memory Dorkus there was a Rage Shrine in Kordar (maybe Ardan's Peaks), one in Demonis and one in the Land of the Dead. These shrines add 5 rage permanently each, but I know that all of the shrines etc are randomised in the game.
Obviously I would prefer 80 Rage to 40, but it depends on what you aren't getting instead. Admittedly I tended to ignore the ragebox a little towards the end, but I still used it for Glot's Armor and Reaper's skills. It's basically a matter of opinion as to whether Leadership or Rage is more important to a Mage, as it would seem that both strategies work. Toward the end when offered Leadership it was over 1000 being offered, so it's probably more than just the difference between 20k and 19k overall. Plus, my strategy involved massive stacks of Sprites, Lake Fairies and Dryads, which are very powerful. You just can't discount the damage 2000+ Sprites can do in a single hit. I have seen 1800 Sprites do 18,000 damage (I think 10 damage per Sprite must be the highest damage they can do on a Critical). Because nothing retaliates against Sprites, I used Glot's Armor on them a lot to decimate massive enemy stacks. Say I hit the enemy for 8000 damage, no retaliation. Then I cast Glot's Armor on the Sprites and the enemy stack attacks my Sprites, breaks the bubble but does no damage, and then the Sprites retaliate for 6000 damage. These aren't criticals either.
I should say that I always have Anga's Ring equipped. Best item in the game, IMO.