Anyone knows how to get legal steam keys when not available on autorized storefronts?

no

With the sales going on at the moment the game might actually be cheaper to gift on steam than to buy a key. trusted or non-trusted seller.

What games?

https://gg.deals/ and https://isthereanydeal.com/ are to useful to find alternative sites to the common ones that sell game keys. Best of luck with finding the games you want to giveaway.

You could try to contact the developers and ask to buy keys from them directly... though I don't know how successful that usually is.

Or you could wait for a major Steam sale (Winter/Summer) which is long enough to comfortably make the giveaways and gift the games via the Steam Store. That's what I've been doing lately. I wouldn't risk it with public giveaways though. D:

Depending on your location and currency, direct gifting through steam is always a valid choice. If you're one of the higher priced countries that basically guarantees you'll be able to gift anyone, so no worries about any pesky region restrictions that sometimes with keys.