I am posting this because it was very difficult to identify the part number for the 'Kit, Left & Right Pedal Spindle Nut, Rally'. It is: S00-01676-00
The good news is that it exists even if you cannot order it direct from Garmin and (in the country in which I live) the Garmin distributor could not find it on the Garmin system.
The bad news is that the kits costs £43.20 (including UK VAT). That's £21.60 for EACH nut.
At that price, it's no wonder Garmin does not advertise it and makes it hard to replace.
Be warned that the RH nut (black) strips very easily.