Well if you are looking for GM parts you are going to need to find the appropriate catalog the car. Once you know the part number, half of the battle is over. Chances are that some parts are still available but they may be of a newer design. They are places that sell NOS (New Original Stock) parts, then there are swap meets.
You are not likely to get much help from dealers because they won't be in the computer lookup for a 1966. They have to go to old books if they saved them and do it manually. Then they know you will grumble on the price after all this work so they would rather not even bother.
I was bugging my local Chevy dealer and I could tell he did not want to look. He gave me his parts books and said here, look up what you need and call me, I will run the part number and see if it still good etc. Found some parts for the '70 El Camino but many were discontinued long ago..
Here is a good place to start;
PARTS VOICE
