In case you didn't know, copper boulders are actually a lot larger than what you see on the surface. After picking at the copper boulder, continue downward into the ground. You will find that copper mining will take out an entire area, leaving a huge crater in its aftermath.

Besides that, it's quite randomly generated along with the world. So far, I've had very little trouble finding copper ore in the Black Forest. They usually appear somewhat close to each other, about a few dozen feet apart. They usually appear more in the depths of the forest, than the outskirts. I don't think I had ever found any in the outskirts.

Also, they are at times hard to spot. Basically, they look like normal stone boulders, except upon closer inspection, they have subtle brown-yellowish veins on them.