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Humble/Fanatical/Indiegala bundles

Humble/Fanatical/indiegala, but I usually buy also from Instant Gaming if I really want a game.

HumbleBundle, Fanatical, IndieGala, GreenManGaming, Gamesplanet, Gamersgate, Voidu, Chrono.gg, WinGameStore, Razer Game Store, GameBillet and Gemly.
Those are all Stores which currently come to my mind!

I recomend to use Fanatical, Humblebundle, chrono gg and if you are doing short giveaways, around 1-12 hours. Gamivo. Gamivo is re-seller site, but so none of my friends have had any issues with it, they said that customer support is very helpful as well.

In addition to all the places people have listed, it's worth mentioning that you should always check for any region restrictions on the keys you purchase before making giveaways for them. It'll save you headaches in the long run.

As far as the best times to purchase, check the Deals section often here on the forums. There are already threads for Chrono.gg's and Fanatical's daily deals, Gamivo has its own thread, etc., etc., and you can also keep an eye on https://www.reddit.com/r/GameDeals/