Side Molding Clip 66' Dart

That looks like the same clip used on the 66 Valiant Signet side trim, if not the same one then very similar.
I have not been able to locate exact replacements.

Some of these that have access for a fastener could be replaced with these
http://m.ebay.com/itm/370543338058

I had a parts car that was using some of these:
http://m.ebay.com/itm/190835517014

Hope this helps.