Check out Titchy Train Group, HO scale, NWB & riverts, they come in plastic,from memory 20 to a card. I think I got the .030 size, I just sent him a sample [KW sleeper door] and asked him matct and send.
As well as riverts there's bolts,washers, hex head bolts etc. gave me excellent service, to Australia.
You mention that you're going to need a ton of them, good luck drilling all the holes in a straight line to fit the riverts into