My 2004 4-cylinder Frontier was without the long pivot bolt for the alternator when I got it in 2011; I discovered this by accident, and this explained why previous owner had receipts showing different size alternator belts being tried from a repair shop.
Anyway, that long pivot bolt is 10mm diameter x 130mm long, had a special locking nut used with it (have that on my '98 Frontier with same engine). That likely will be a dealer or special-order item, not Ace Hardware or auto parts stores had longer than 120mm (D' oh !!!).
Anyway, I bought and used a 3/8 inch bolt from Ace, along with a nut and lockwasher instead.
For the other two alternator mounting bolts, those thread into threaded bosses so those must be metric. Ace Hardware or aurto parts store typically has better metric selection than Lowes or Home Depot.