Sometimes it is hard to find brushes tiny enough for painting features on a small scale doll. Look in your local hobby and art supply stores for Ultra Mini brushes. There are on-line source too; try Jerry's Artarama, Dick Blick Art Supplies and Micro-Mart. I'm sure there are others too.
We have found that synthetic filament works as well as natural hair with both acrylic and heat-set paint.
Not familiar with brush sizes? Ultra Mini sizes are labeled 10/0, 12/0 down to 20/0 (one of the smallest is 20/0 and is very hard to find).
Some shapes to look for are Filbert, Round, Spotter, and Liner.
Want to know more about art brushes? http://www.misterart.com/store/browse/001/cat_id/8/Art-Supplies-Brushes.htm Scroll down the page to read more about brushes.
Prices vary considerably from supplier to supplier. Shop around!