Man I really need to go out and measure the size of carriage bolts I used in mine. They are stainless steel and I bough them at Ace, total I think I spent less than $2.00 on the mods.
I see you have a silver bumper, so do you have the sport package? If so, then you may need to just get some small ass carriage bolts for those little holes in your bumper. Perhaps paint them silver to match as well.