Rear Belly Pan Flapping In The Wind

I was driving on the freeway last weekend and suddenly hear a loud thumping/flapping sound coming from the rear of my Variant. I pulled over and the rear hatch was latched, but not totally tight. I reclosed it, thinking I had figured the problem out. I get up to speed on the freeway and it starts again. I pull over again and start looking for something else that could be causing the problem. I looked underneath and apparently two of the three bolts that hold a circular belly pan-looking piece are off and that circular piece had been catching wind, bending down and scrapping on the ground. Are those bolts available at the dealership or is it something that I could get at a local auto parts store? I've never had issues with bolts, so I am not sure if anyone would even carry little stuff like that.