Where to buy keys?

Also, can I create a giveaway, select the Gift option and deliver the game by giving a gift through steam instead of a key after the giveaway ends?

Of course you can. That's the reason why "Gift" option is there.

You can buy keys from a locksmith or a hardware store.

This topic may be useful for you: https://www.steamgifts.com/discussion/Yd0M4/list-of-official-licensed-steam-retailersstores

Beware if you buy games on steam. You can't gift games to more expensive regions, so make sure that the you exclude those regions when making the giveaway.

Home · SteamDB · Steam Database
This is useful for checking the information on "Regional Restrictions" and "Price Range" of game keys.

Hm?
I am not sure if I can explain well.
If you do such a thing with the public GA, it may not be possible to get rid of the GA if there is a problem.

Create "off topic" in "Discussion". Indicate "SteamKey Transfer Instructions" and post "If you have any problems, please agree to the deletion and participate!".
Post "Invite link".
It can be handled smoothly when a problem arises.

When buying outside of SteamStore, I feel that the key for the buyer's area is given.
Therefore, at the time of purchase other than SteamStore, it may be purchased using the gift function etc. of Store side and transferred.
In that case, a key is issued according to the area of ​​the person who opened the gift link.
(Not all Store is such a mechanism, but there is a tendency.)
If the key still does not work well, you may be able to exchange the key if you leave it to the "seller".
If it still doesn't work, it can't be helped?

Fanatical
Nuuvem
Greenmangaming

https://isthereanydeal.com/

Lists legit resellers.

http://www.cheapshark.com/

Thank you all!

Kinguin
Cdkeys

www.dailyindiegame.com (just stay away from their bundles, waste of money)
www.steamground.com