When I did my bumper, I changed the bolts to ones about a 1/4" longer to compensate for the brace. I got the bolts from an Ace Hardware store. I don't recall if they were standard or metric but I just got the same grade as what was on there. No special dealer part needed.
If they are an SAE bolt (Seems to be some disagreement above) then I would highly suggest buying some F911 bolts or a Holo-Krome SHCS instead of the G8 junk.
Something of both a higher strength and of greater ductility is called for. G8's only offer a reasonable strength, but their YTS and UTS are so close together (indicating low ductility) that when the strain exceeds the YTS by not much they will fail.