Usually off another 1965 Impala, Biscayne, Estate Wagon, Caprice, Bel Aire. There are also compatible sheet metal found on Pontiacs, Olds, Buicks, and Cadillacs from the 1965 model year. Everything forward of the firewall is the same on all of these cars in terms of sheet metal. You need to invest in an old (out of priny) Hollander Interchange manual to find the parts off donor cars that will bolt up. They come up every now and then on the internet for about a hundred dollars a copy.
Here is one that is too new for you but is illustrative of what I am talking about:
1965 Impala front fenders are not reproduced, You can buy patch panels for the lower sections. As for interiors and dashes , check out some of our sponsors and also look in the classifieds here on this site.