Here is a new list with all the cloud shapes available for free on the internet that I could find so far. As you probably know, a vector cloud is useful if you want to create cartoon illustrations, icons, website templates and so on. For photo manipulations, where you need more realistic clouds I recommend you to use cloud brushes or cloud textures.
This collection contains over 50 cloud shape vectors in CSH file format, so plenty to choose from. You can easily change the shape of these cloud vectors and combine them to create new and interesting shapes. You will have to use the Photoshop vector tools to do that.