For games that are no longer supported or sold by their original developers, sites like My Abandonware are standard.
Be cautious of websites that ask you to complete surveys or download "installers" before giving you the game file, as these are often scams.
Many small-scale developers have released games under this name on platforms like Itch.io or GameJolt. These are often free to download in their entirety because they are passion projects or game jam entries.
When searching for "free download full version" for PC games, it is important to use reputable sources to avoid malware: