This applies to the United States (at least):
In the U.S., the third (white) wire is NOT ground it is neutral. DO NOT ATTACH THE ESD WIRE TO THE THIRD WIRE IN YOUR PANEL as suggested by another post, without consulting a licensed electrician or registered professional engineer qualified in residential electrical systems registered in your state.
For informational purposes only, "The National Electrical Code," written by the National Fire Protection Agency, requires that on three wire systems (which it is assumed you have since you do not have a ground connection in the panel), the neutral conductor may be grounded with a local grounding conductor not smaller than the neutral conductor entering your service panel using a 3/4" trade galvanized pipe or a 5/8" galvanized rod, or a 5/8" copper rod, all not less than 8-feet long, or a two square foot plate electrode at least 2-1/2 feet deep or a water pipe with a continuous 10 feet of it in the ground within 10 feet of the panel. Gas lines as grounds are expressly forbidden by the NEC.
For ESD ONLY, keep it simple. If you cannot accomplish establishing a low impedance ground by attaching to the water supply line (at the source outside your house), only then add rods, pipes, or electrodes. If you leave sufficient excess wire in your ESD drain wire to connect to a rod, pipe, or electrode, if necessary, then you have not wasted any time. Only fool with the earth's chemistry or adding water daily or charcoal or the like as a last resort. Again, keep it simple.
If you choose to add a ground for anything other than ESD, then reread the first paragraph.