In this tutorial, I used:
I highly recommend any of the brushes by this artist. They're really spectacular.
Here are some free brushes that look like they would work for this tutorial as well:
High Res Cloud Brushes from qbrushes.net (This link seems to be broken, but you might be able to find some good brushes by googling 'High Res Cloud Brushes.')
This tutorial can also be viewed on my Deviant Art page.