If you are talking about the extender pipe, you can get them at any hobby shop. If you are talking about a cap to keep the oil from dripping out of your exhaust while you transport it, I couldn't tell you other than taking one off an old fuel bottle.
As far as the exhaust extender, I don't think they are worth it since they can fall off or get knocked off during heavy action. And it still wont keep the nitro oil off the side of your car/truck body.