I am using a stereo 3D pen. What surface should I use?
You could use a piece of glass, that's what most people using 3D printers have as a build surface.  An easy source of glass for pen use would be a picture frame but the edges are likely sharp so be careful.  Acrylic would also work and is easily obtained in small pieces from places like Lowes/Home Depot, I used Acrylic for some time on my Kossel.  The plastic can stick to Acrylic very well but I had no issues using it with my printer, just test it out and see what process works if you go that route.