Sorry, I am not aware of a source for just the clips. Just a word of caution that you should not connect long ground wires to the probe anyway: The impedance of a long wire will distort your signals, and the large loop area formed by the probe tip and ground wire will act as a pretty effective antenna for unwanted background noise.
I would not go beyond the length of a typical ground strap (as provided with the probes) -- shorter is better. So the good news is that, if you buy complete ground straps, you can use the C clip + cable and only have to discard the crocodile clip.