The Gordunni Ogres at Gordal Fortress in Talador drop them at a decent rate. I farmed them in groups of 4 or five starting at the base of the fortress than moving up. They sit in groups of 2-3 so its not to hard to pull groups of 5 or 6 at a time.
To get inside the fortress, you need to have access to or have completed the quest: Dropping In
if you are on that quest or have completed it, you can enter the fortress using the rope/portal that is just to the left of the force field blocking the entrance.
Just found this new location which is MUCH better then the previous. Drop rate is very high.
Grimfrost Hill located at around 66 47.
Took me about 15-20 5-10 mins to gather the 25+ stones using this area.
The portals are located in:
Gorgrond at 58.0, 34.5 by Blackrock Foundry
Nagrand at 32.2, 46.2 by the Ancestral Grounds
Spires of Arak at 54.6, 51.6 by Veil Zekk.
Talador at 55.1, 48.1. Thanks "decompiler" for the edit.
Frostfire Ridge at 61.5, 45.6 by The Crackling Plains.
Shadowmoon Valley at 50.5, 35.7
It takes a total of 25 stones to unlock a portal. You can have a total of 3 portals in a level 3 mage tower.