You can take old coax and strip the center conductor wire out, then flatten out the ground braid and use that... I've used that, and also on ebay you can find ground braid usually sold in 10' lengths... never had much luck finding it at auto parts stores.
How I was using old coax, was I would first strip off the outer coating, then remove the center wire, flatten out the ground braid and stick some solder on the ends... then take self tapping screws and attach the ground wire... using solder on the ends will help the screw from messing up the braid.
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