There are a few US manufacturers like Star, MMS, etc, but most everything is coming in giant lots from Chinese contractors. FNL makes certain items here, but the majority comes from overseas. The US manufacturers typically only make high movement items for government contracts that require US-made materials. So 1/4", 1/2", 3/4", etc that are common standards in new construction.
I had a hell of a time trying to source 1/4" fender washers that were made in USA for an electrical contractor once. They needed about 20 for a tower at a government facility and their contract required 100% made in USA. I had about 600 of them in my warehouse, but they were made in China like everything else. Between a dozen US fastener manufacturers, I got one that was willing to shut down a line and set up tooling to make them within the two week deadline. Ended up costing the taxpayers (you know the contractor was just going to pass the cost on like anyone else would) over $600 for 20 washers. I tried to simply give the standard Chinese ones to the contractor (they were under a dollar, their price anyway), but they had to have a paper trail for every single material, down to the individual washer.
