I got mine off of EBay- but mine was slightly different trim, so I can't give you the exact listing.
You need to get the inside measurement of the trim
You may need to measure at several different places, my '66 trim changes width along the side (wide at the rear and doors, then narrows down a bit for the last 10-12" of the front fenders). Make note of how many of each size you will need. For the sake of discussion, let's say you need 1" clips. Search on EBay Automotive for 1" trim clips- something like this should come up:
But before you order, I had several that were inaccessible from the rear for various reasons so the nut cannot be installed or tightened. A couple in the front fenders that went into the reinforcing rib, and several in the rear quarters that can't be reached through the taillight openings no matter how long your arms are... for those you need push-in clips like these:
For these you have to make note of the hole size as well as the trim size, since they are a press fit. And believe it or not, the hole size does change along the length of the trim... WTF??
A word of caution on these clips- the ones I got were a bit too thick to properly lock into my trim, but a quick swipe across the bench grinder to reduce the thickness was all it took to make them fit perfectly.
So once you know how many threaded ones in which widths you need, and likewise how many plastic ones, you'll have it whooped! I paid about $10- 15 for bags of a dozen, in each size needed.
As far as the C pillar covers go, I ended up bending up my own out of sheet metal.
I have honestly NEVER seen trim made out of such thin, flimsy (and after 60 years, brittle as all get-out) plastic as these things. I had 2 sets of originals that were so broken and brittle that I could barely keep them glued together long enough to make a pattern. I figured even if I could find some different ones I probably still wouldn't be any better off, so I made some out of thin sheet metal. I'll post pics if I can find some (car is stored for the winter).
You'll run into the same issue with the D pillars in the rear.
Best of luck whichever way you go with these.