Basically, you're just looking for PC game stores that carry free games other than Steam, right? In that case, Itch.io is the main place to look. Beyond that, games will primarily be distributed on their developer or publisher's website. Piracy probably won't be an easy option, as people aren't going to put in the work creating/uploading pirate copies of a game thats easily available otherwise.

Edit: Also, I think you may be able to use SteamCMD (Steam's non-launcher, CLI tool) to download the game if its DRM free.

Something like this in SteamCMD: