Bicycle brake cable. If you need something fatter, then automotive speedometer cable, though I doubt you will be able to wrap it around any scale spools. Another option is lanyard cable for race car hood pins. What you are looking for is called wire rope but very SMALL wire rope. You could also try kite string and dye it grey. It definitely would be flexible enough to spool. Another option would be heavy duty thread for sewing canvass tents and awnings. Good luck on your search.