When all else fails I usually grab my bolt bin drawers and try a few, except those are mostly standard and they are probably metric so I'd head to a hardware store and get a handful of likely samples.
Except, I'm wondering if the ones in the top are the same size so you just need to pull and measure one of those or compare at a hardware or auto parts store?
Will more bumper work be needed soon? I'd also consider using rivets if that was the course of least resistance and they'll work. At this point I'm just makings things up, would have to see it to know for sure.
