The nut is a standard M8. I know this because I only thumb tightened my alternator when I reinstalled it. It of course vibrated loose and though I retrieved it months later from the foam above the stiffening plate I needed it to work.

You won't need to get a replacement before you eventually do some work requiring stiffening plate removal. A normal M8 nut with a washer is a permanent fix. The only thing special about the OE part is the outside is plastic so you won't short out the tool used to install it. Should go without saying be very careful installing the non insulated nut: I put a rubber hose over my wrench.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)