rivets

In the drainage tray above the battery, there are fastening clips which are push-in rivets to hold the tray in place. There are three of them. I'm missing one. I've been trying to find out which types they are, trying to order them on ebay. For example on...

Can anyone tell me the best way to remove these? Swapping number plates over at the weekend and one of the screws that go into the threaded rivet had rusted solid and would come out no matter what I tried. Ended up drilling the thing out and buying a new rivet from stealers to replace the...