Interestingly the actual impedance doesn't really matter. Any two-core plus overall screen cable is going to be in the 90-130 Ohm range and any of them will work. Likewise, for a termination resistor, anything in the range 100-150 Ohms is way superior to having nothing. And, for DMX, it's not that important to match the resistor to the cable.
The reason for the latter is that there are so many discontinuities the overall impedance is rarely a single figure.
What is importance is, as David says above, the cable's capacitance.