If people make brushes and publish them, why would they want to restrict people from using them? Cloud brushes are the easiest way to get some nice cloud effects. Stocks are also great. Or you can try smudging.
you can make them via digi painting them or you can use a hard brush, and smudge, I just dont know the settings
Filter>Render>Clouds ....(it will use what ever colors you have set as your foreground and background[pressing CTRL+F after that will give you different results]) If you need a little flow to them you can always smudge them a little.
depends on the type of clouds you want. personally, i use gradiesnt and smudging to create clouds. techniques vary.