How to get game keys? (wanna start a giveaway)

Bundles, trusted resellers, or if you want to risk your money, the grey market.
But the vast majority of the GAs here are from bundle leftovers.

You buy a key (e.g. greenmangaming or gamersgate), have a key left from a bundle or a gift from Steam. You can buy game keys and gifts from traders (e.g. trades) but might run into scammers, be careful.
You can not give away keys from your Steam library!

Okay, thank you guys!