In August 2012, I attended a wonderful day of classes at the Susan Carberry Cake Cottage. The courses introduced everyone to One Stroke Painting and how to use these techniques on cakes and cookies.
The paint brushes recommended by the instructor can be purchased many places. I've attached a link to the Amazon site so you can see the brushes and read the reviews:
Although these brushes are specific to the "one stroke" technique, they are very nice quality and were recommended by our instructor to use with the gel colors on fondant.
Here is a link to Susan Carberry's FB page with an example of the winter class projects:
Here's another link with the cake that inspired me to take the class and buy the necessary supplies: