IMHO the chances of finding a windshield worthy of a concours restored car is slim to none. Most of what you will find are scratched and pitted and on top of that you will be looking for a correct date code. You might find one good enough for an unrestored car but not a restoration. You can polish out some of the small scratches, but the deep ones and rock nicks will not come out. You might find a good back glass if you find a southern car that did not get frost scraped off hundreds of times. If it were me, I would get both new and put my efforts in another part of the restoration.