You can get UV marker pens of assorted fineness on amazon. There's a few clear nail polishes that are UV reactive that work quite well as UV paints. A lot of UV paints are actually phosphorecents that have a base colour that then gloes in the dark, or are white - so you want the invisible stuff and that tends to come down to nail poishes and security markers.
Some of the marker pens come with a snall UV emitter on amazon so you can show off your handiwork...