I believe the rivets you are referring to are located in the inside area of the wheel well that secure the front cladding as well. You really don't need to replace the rivets as they need to be replaced again if you plan to remove the cladding again in the future. The cladding is fine without them as they are held onto the vehicle with other security components.
However, if you need to replace them without question, take the ones you drilled out (what's left of them) and match it up with any you find on the market. You can also take the drill bit you drilled them out with and bring that as reference as the hole you create will be the same size for the rivet's shank size.
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