The website is Brickinstructions.com, and it’s basically a giant online library of official Lego instruction manuals. Lego themselves have a massive online library of building instructions, but where theirs only goes as far back as the late 90's, Brickinstructions.com’s collection contains scans of instruction manuals from as far back as 1964. You can search for instructions by year, theme, or set number, and although they don’t have instructions for every single Lego set ever made, they do seem to have most of them, or at least a significantly large number of them.
Just as an example, here’s the instructions for the 1913 Cadillac set from 1975:
Advertisement
Enjoy!