I'm pretty sure it was Ted Cochran who discovered these stainless steel clips many years back, and he eventually got NARTS to resell them. They're made here in Minnesota by Lakewood Products. You can order them direct from Lakewood, but, last time I did so, they had a minimum order of 100 clips.
https://www.lakewoodproductsinc.com/products/alligator-clips/
When I went to solder them, I stopped by my local hardware store. I found one of the "old timers" who works there and described what I was trying to do. He disappeared into the back room for a while, and then reappeared with a dusty bottle of flux. "Should work", he said, "Been back there for a long time. Wouldn't know what to charge for it, so you can just have it." (Of course, I was a regular customer there - known as the guy who bought stuff to fix pinball games.) I love local hardware stores.