Amazon offers the e.l.f. Cosmetics Expert Liquid Liner 0.15-oz. Bottle in Jet Black for $1.50. Check out via Subscribe & Save to cut that to $1.43 and bag free shipping. That's about a buck less than you'd expect to pay in local stores. It's smudge-proof and features a fine tip brush.
Note: Be sure to cancel your subscription once your order ships to avoid being billed for further shipments.