how to purchase a copy of a game to giveaway

Purchase a key from a site like Humble, but make sure you can give it in the regions you want to, some keys are locked to certain regions.

Gift copies are no longer an option for most games. The only way to get them now is purchasing a 4 pack or similar on Steam, they give you the rest as gift copies in your inventory.

If it's on sale, make a giveaway quickly as possible, end it as quickly and pray he/she comes online quick enough to accept the gift, though you can set some restrictions that they will (still no guarantee) in short it's a risk.

Also some countries or something can't accept gifts from other countries because price difference, but from europe (guessing we are the most expensive) i never had issues sending to other countries.

Safest option (but not without problems):
Search through indexers like gg.deals or isthereanydeal.com to find the cheapest available option for you. Be aware that Steam Keys can be region locked or may not (you can see in @Lilly;s threads how for example HumbleBundle gives keys for regions.

Very limited option
Buy the gift after the winner is known. But before you create GA you need to know that you cant gift for other regions that have price 30% bigger or lower than yours. For example for GTA5 in my region would be img

It is best to buy a key from a reputable site like Humble bundle, Indie Gala, Fanatical, GMG, and others. You can still buy the game direct from Steam, but since you can't buy games to store in your inventory anymore, you have the purchase the game when you are ready to gift it to someone. With giveaways on SG, the winner has 7 days to accept the win, which means if you want to buy a game on sale, you need to make sure the sale will last past the 7 days incase the winner doesn't respond quickly. Steam has also added restrictions so that you can't send a gift to someone if the price you pay is a certain amount cheaper than their region (not positive, but I think it's around 10%). These things make it fairly difficult to gift directly from Steam now which is why most user have switched to just buying Steam keys from reputable sites to gift.