A possible source of rubber legs is bungee cords, some of them have white and black in them. Elastic cord are also good sources, of course then don't come in the color selection available at the fly shop. You can add color by clamping one end of the rubber in something and then clamping something like forceps on the other end and twist it. Pull it taut and run a felt tip down the rubber. When you release it the cord you have a stripe around the leg.