Don't know about on line, but virtually any hardware store. In general though it has to fit within the tripod and the camera, so you need more that just the size of the thread.
...Unless you are in Europe, in which case you will need to shell out the money for a "special camera one" (special because only camera stores sell them outside the US) from almost any camera/vide/tripod store.
Or find a friend in the US to send you a bag from their local hardware store!