I have both of the Rhino's with me on the road and i can work on them when I have a few minutes to spare….. and a big table of course….
The pen I use for the fine black illustration is a 0.5 uni pin by Mitsubishi.

However, I also have other pens…
Prismacolor premier 05 pigment markers they come in 9 different colors.
I purchased them at Hobby Lobby, I've not seen them anywhere else…

I also have Faber Castell landscape pens, 6 colors and they are a fine brush. I also got this set at Habby Lobby.
I also use prismacolor pencils…. particularly for the eyes.