You could try Auveco.com. I bought several pounds of bolts from them when I rebuilt my car. They also have body clips for chrome molding and every other manner of clip or fastener that you might need.
I have many good used bolts for most applications on our 63/64 Cadillacs. Just packed and shipped out an exhaust manifold bolt set.. very unique of you want to stay original. Just a wild guess but from parting out 52 of these cars i would guess 200 to 300 different and unique types of bolts/screws that hold these cars together. If you can't find what you want new, shoot me an email to firstname.lastname@example.org and indicate exactly what you need.
As far as trim fasteners, I have went through the "trial and error" process of figuring out what some of the main fasterners needed are for our cars and have new sets ready to ship. The lower side trim on Series 62 and deVilles are one example which you can see by Clicking Here.