Where do people get Steam keys from? (Cheapest)

If you want quality games at a reasonable price --> Humble Bundle 1$ Tier

if you don't care about the quality of the games and want to do something nice for card farmers --> BundleStars 1$ Bundles, GoGoBundle Groupbuys, SteamGrounds, IndieGala and many others

I think Humble Bundle is the best place

http://store.steampowered.com/

https://www.humblebundle.com/store
https://www.indiegala.com/
https://www.bundlestars.com/en/

https://www.g2a.com/

This thread is constantly updated with the latest game bundles around, all from official sellers
https://www.steamgifts.com/discussion/C0hpX/new-master-list-of-ongoing-bundles

Thanks a lot people!