High quality empty, refillable markers with replaceable tip.
Fill with almost anything of a reasonably flowable consistency to make your own customised marker. Try with acrylics such as the Golden Fluid Acrylics (for an acrylic paint marker), Dye-na-Flow (for a fabric ink marker) etc.
This marker contains a metal ball-bearing for mixing and shaking if colour has settled. Pump action to start the flow.