You can buy them at Sephora for like $25, but I found a handy video on how to make on yourself. You can see it for yourself at the bottom of this post, or here. So I made one myself and here is how it turned out:
Basically, you buy cheap thin paintbrushes (I got mine at Walmart), but it must have a metal ferrel. Then all you do is bend (about a 45 degree angle) the top of the brush with a pair of pliers and voila, you have your very own (cheap) angled eyeliner brush. Watch the video to see more details and also how to apply the eyeliner once you have made your handy brush.