Identifying non-standard flange fastener/nut
That is nut is 1/8&quot; pipe thread, which is what standard lamp pipe and fittings are. Any lamp shop (or even a decent hardware store) will have a variety of lamp thread fittings available, including lock nuts. I call it lamp thread because while it is 1/8&quot; pipe thread, the pipe and fittings are not tapered like 1/8&quot; pipe thread plumbing fittings are.
While you may not find an identical flanged nut like the example in your picture, it would be easy enough to use a regular lamp thread hex nut and use a washer to increase the surface area.