I recently had the problem of trying to find one of these things that screws on to a camera shoe-mount. I went into numerous camera stores, pointed to the part and asked what it was called and if I could just buy that part. Nobody could tell me what it was called and in all cases I would have to buy a $10 to $20 shoe so I can just get one or two of these pieces. I needed at least 5 of these. I couldn’t even figure out what to google.The only thing I knew for sure was that the thread 1/4″ – 20 (this refers to 1/4″ diameter and 20 threads per inch).
I found that searching with google images helped narrow down the search. I finally found a company called Drillspot that sells a “Check Nut, Knurled, 1/4-20, Steel“. Bingo. I would prefer them to be a bit wider but this will do. I also found another company that sells plastic versions in bulk. They are cheap but too small to really use in this context. I ordered a few of the steel nuts for my project. Yeah, shipping on these adds up but that’s how it goes. Buy them from Drillspot.com here. I’m sure you can find these elsewhere. Good luck!