Mcmaster-Carr has every metric,US standard lefthand nut you could want for,like.$6.00 a box so you could buy several sizes/thread pitch if you dont know exactly what you need
Another alternative would be a “push nut”. The are made from a spring stainless steel and are not subjected to thread pitch or direction. They are not quite as sturdy, but they don’t back off and you can always use two of them or put a little epoxy or JB Weld behind them. Google Fastenal to locate an outlet near you.
Joddy! Give Fastenal a call in Minot and they will have your left hand threaded nut. They can just mail it to you. Of course, the machine shop idea suggested on the show will work too!