Looking for a Gundam Marker alternative? Try the Uni Posca line of paint pens made by Mitsubishi Japan. The operative word there is "paint", that's what comes out of the pen, not ink. Available here.
Works well on plastic and comes in different head sizes. I bought white, black and silver. The white and black ends up in a Matte finish when dried, which is the effect I want. While the silver one has a sort of metallic glittery sheen to it. Why I consider it good is because it applies easily on plastic, like a Gundam Marker.
I got mine off of ebay, but I see that there are some selling it on Amazon. Be sure that you are getting the correct paint tip size that will fit your needs as there are many variants available.
The smallest tip size they have is 0.7mm. It's a felt tip and I don't think it's small enough for panel lining.
