Gentle pull a good one out to check size and shape. You can then buy a generic fastener that looks similar. while at the auto parts store get a set of plastic/vinyl tools made for removing those clips and other body panels without damaging the under parts. You pull the center piece of the fastener out first. Do not get behind the entire face of the fastener and try to remove it. Pull the center piece out which will allow the tabs on the inside to reduce in diameter and then work the fastener out of the hole.
I just removed one from my car. The fastener is actually two separate pieces the large piece will fit in the hole first then the center piece will slide into the first and expand the tabs on the plug. The diameter of the first piece, the plug is 7mm. If you have any silicone grease use a dab on the plug and the slide as it will make removal much easier should it be necessary sometime in the future.