If you want small thorium veins in good supply, head to the burning steppes. There is a mountain overrun with ogres there. You can find lots of small thorium veins in the caves there.
For Rich Thorium there are two spots in Winterspring that have about 5 nearby spawn points. Every few minutes one of them will spawn. I recommend the Furbolg village northeast of Everlook. It also has I think 4 treasure chest spawn locations in the same area. It's a great place to pick up treasures. You can also get rep from killing the bears that infest the place. You just need to get a certain reputation level with a certain faction (I can't remember which) to do this.