Your Traxxas truck should have, in the manual, exploded views that show each part w/ part number and where it fits together. You can find these exploded views online here:

http://www.traxxas.com/products/nitr...tler_views.htm

Now after you find the part #, to purchase the part you can go to Tower Hobbies like suggested above or you can go to http://www.buytraxxas.com/ and simply type in the part # and purchase your part from there.