I wanted to do some detail veining on the eye lids of my first doll, but couldn't find a small enough brush to do so. I went to Michaels craft store and they must of had a sale because all the small brushes were gone. I was looking for something that had like five brisles or something that was thin but could find nothing out there.
I always use a liner brush, usually 1/0 or 2/0. We do carry a liner brush on our site: http://www.secristdolls.com/inc/sdetail/22032 If you need to you can always take a small sissor and cut some hairs off or even a nail clipper. I find that the liner brushes are great because I only have to load it once for both eyes (you get less random dots that way)