Rean might work. Neutrik made them also, theirs might work too. (or did they buy REAN?)
Depending on age, they might be from Cliff.
More recently, probably some chinese source, I have no idea who.
I don't know if any hardware store nut will fit it, I never measured. If I needed one I just went and got it out of the drawer, so the question never came up.
Yes I USED TO work for Ampeg...but I haven't forgotten everything.