You mean the M6x1 locking nut? Just about any auto parts store (autozone, napa, orielly's) will have M6x1 locking nuts. Where are you located?
I wonder if he's talking about these? If so, I would say Ferrparts. Maybe Ricambi has them. Image Unavailable, Please Login
Get used from a parts supplier like Ferrparts or equivalent. get the part number from any of the online calalogs.
Ricambi or Ferrari only show the whole tail light as an assembly on table 140. I suggest you take one of the ones you have to a machine shop and have one made. If it were me I would machine one myself.