Been looking for these as much as I can. Here's my current list:
1. Borean Tundra - Steam Clouds in the Gyser Fields.
2. Northern Howling Fjord - Artic Clouds in the snow areas.
3. Dragonblight - Frozen Spheres in the north area where the harpies are. Have reports of "fire" clouds in the Obsidian Dragonshire also.
Please feel free to add to this list... I have hopes we will see other cloud types soon... I have high hopes of finding a shadow cloud.