Perhaps it's worth noting that, as is far too common with business web sites, only the email address gives any obvious indication that this is a UK-based supplier. There's no visible "store-front" address, and no obvious invitation for orders from "foreign buyers," (i.e. US?) although they may be able to work them.

The example parts appear to be all metric threaded, which probably is pretty much universal for european equipment of the kinds suggested and appears to be the prevailing kind on "sound equiment" sold now in the US; but I have encountered inch-thread fittings on some older equipment in the US. This supplier may have other choices in their full catalog, which seems to be available.

In my part of the US, inch-thread knobs, male & female threaded, are fairly easily found in a few styles/sizes at some Ace and True Value hardware stores, once you figure out which individual stores have them. Some carry a few metric variants, but except for a few common styles they're much harder to find and fequently not in stock. The supplier that the US sellers get their stuff from probably could be easily identified for direct ordering, but I haven't had a need to do so.

I haven't found the "music stores" who sell the stands to be of much use when parts are needed, but there are still some in my town where I haven't "established a relationship."

John