What country are you in? you should be able to get these at any hardware store
I do’nt think they will be slim profile like originals ?
you’d have to look. Go to either a local hardware store or a fasteners specialist
3/8 locknuts are pretty easy to come by. Hardware store, online, etc.
But those speed rings 
. For whatever reason my Trampa didn’t come with any. Couldn’t find anything that came close, ended up using a 3/8th drill bit to drill out the inside diameter of existing washers and a dremel to grind down the outside diameter.