From what I recall, they are the same as those used on the Chevelle and I found the exact ones at my local hardware store in the "Sharon Hardware" pull out trays. It requires a special tool to "roll" the back side of the rivet but they can improvised. The ones I found are dead ringers and are simply a pan headed, hollow neck rivet. I can try and take some pictures later tonight.
Here is the link @wot was talking about.
TCS mounting variations
Rick