Can't help with the screws, but the 'nuts' are called speed nuts, you can pick them up at Lowes/HD or most auto stores. For those random screws/bolts, I polish them up with a wire wheel on a bench grinder, saves some time and $$ and they come out with a nice shine and clean threads.
Thanks Tom - not sure why I was blanking on "speed nuts" - there are quite a few of them used on my Triumph. I suspect I may be able to go a little thicker than original on these and may be able to find some pan-head stainless screws that will work. Won't know until this weekend when I get a chance to run to the local hardware store. My local Ace Hardware hasn't let me down too often when I need oddball sizes of fasteners.
Unfortunately I can't clean these originals up. The rust monster has eaten into them to the point where 2 of the 3 are nearly rusted through. The one in the pic is the best of the 3 and the threads on it are not great.