Finding the right eyeliner brush can be tricky, especially if you're looking for something with a really fine point for a precise application of cream or liquid liner.
The Louise Young brushes come highly recommended (and I do covet them, along with pretty much everything else from that brand), but they may not be in everyone's budget.
Your local artist supply store can provide some great alternatives, however. Take a look in their painting section, especially the synthetic brushes they have available. They literally have several dozen options in terms of thickness and slant. I've been using this one:
The best part? It was about 2$.
(I purchased this item.)