They are called different things by different people, scrivets, plastic blind rivets, *insert body part here* retainer, push type rivet. They will usually have a size associated with them pertaining to the hole they fit into.
I couldn't find any suitable ones online anywhere so I went to the local Holden dealer this morning with the shroud which helped the parts guy find the correct ones using the shroud's part no. He was probably the friendliest / most helpful parts guy I have come across in a while. They didn't have any in stock (apparently there are only 15 of them in the state) so he has ordered four for me and will have them in on Monday.