Wether I am painting hair or shoes I use Taclon paint brushes. The bristles are very soft and flat, this ensures you get a neat stroke with no 'feathering' The brushes come in many different sizes I mainly use a 1/4 inch and an 1/8 inch and very fine tipped.They are available at Riot Stores.
* I have no monetary links to the products above, I simply find them the highest quality and easiest to use