If you have a Tombow multi-pen, we offer Monteverde liquid ink refills as replacements. They write very smoothly and are available in eleven different colors.
If you have the new Tombow Centential Ulta mechanical pencil. It fits .5mm lead.
Have a Tombow fountain pen? Tombow fountain pens fit standard international-sized ink cartridges available in assorted colors. If you have the ink converter that came with your Tombow fountain pen, you can then reload ink from a bottle. Colorado Pen offers a huge selection of bottled inks that will work quite nicely.