they are not bought on steam but on resellers, like humble bundle, indiegala, etc.

On Steam you can only gift games and some are region locked. Also the prices can lock you from gift another country.

For keys check stores like Fanatical, Humble Bundle, GreenManGaming.

You can't purchase keys for games on Steam, but you can on a couple of other websites. Humble Bundle and Fanatical are popular websites, so is Indiegala, GamesPlanet, Greenman Gaming and GameBillet. Alternatively you'd have Keyshops which are sold by other gamers or something, but they are quite the gamble when it comes to it's security.

I've had no severe issues with them, but you gotta be really careful. They are a lot cheaper and you can get out some great deals, but again, that's not my best recommendation when it comes to the safety of your giveaways.

When it comes to Steam, well you can choose to contact the owner or he needs to contact you through Steam. You have to be friends there and you can choose to send him a gift through there.

Fanatical and Humble are the best bet — they're official resellers with a good rep. It's safe to buy from them, they won't revoke the keys you've bought if you won't redeem them for too long, and they sell good bundles from time to time (but, yeah, there weren't many interesting bundles lately). You may also sent the game to the winner as a gift via Steam. But keep in mind regional restrictions for the keys/gifts— it can be tricky, especially for those who live outside the US.

Steam only lets you send a game directly to a friend when purchasing it. So be warned that winners might not be around to claim it straight away and the sale might end before that. So for steam sales it's best to get in there quick for giveaways.

[They used to let you send it by email or keep a copy in your inventory to send later but they stopped all that]

As said there are key sellers like humble bundle, fanatical & indiegala. Those are fairly safe options. There are other sites like dailyindiegame but those are the big three.

Might be worth knowing that Humble Bundle doesn't really like people gifting keys. There stance is basically 'gifts are for close friends and family'. There isn't really a way for them to know who redeems a key, so it's not a big risk. But they have banned people for that or for buying too many copies of the same game (not sure what number 'too many' is). If you're in the EU you at least have right to any keys you already bought. If you're in the US and haven't redeemd them/written them down then they are lost. Again it's a small risk, just worth knowing.

The bulk of us are likely subscribed to Humble. We keep the ones we want and give the rest away.
Indiegala & Fanatical also have bundles that may contain items we want. Again we keep the ones we want and give the rest away.

There are others, Those are the main ones that I keep an eye on.

Welcome to Steamgifts, enjoy your stay.

🌞

To put the emphasis only on Fanatical and Humble Bundle is counter-productive for a functioning market. There are around forty retailers selling Steam keys (not counting grey market sites). For an overview look here: https://gg.deals/deals/ you might also use ITAD https://isthereanydeal.com/ but I find ggdeals has the better interface.

As was already said gifting on Steam directly is pretty much dead although it is still possible in theory but region restrictions and the inability to store gift items in your inventory as in the past killed it.