Of my 3 SJCAM cameras I had a case, where fastening nut of one bracket was not threaded and it was impossible to use this bracket. Fortunately those nuts are cheap and easy to replace. What do you mean by "One of my brackets does not work"? Maybe the reason is a faulty nut?
Yes, it is a faulty nut. But that was not my question. If there easy to replace then where can I purchase them??
You need an acorn (dome) nut size M5 with 0.8mm pitch (DIN1587), preferably stainless steel. Easyest way is to visit local hardware store. But you can buy online for ca 1.5 USD per 10 or so. Google "DIN1587 Acorn Nuts M5 stainless steel 304" For instance: https://www.ebay.com/itm/M3-M12-Dom...less-Steel-G316/272991181402?var=572211778212