I think when I put mine on (my car didn't have one when I got it) I went to the parts store and found some in the help section. It's just a black sleeve type thing, and you push it through and push the pin in to open up the backside.
Originally they may have been held on by a wire type thingamajigger. :D
Found a pic. They call them push pin rivets, that's what I used.