Verified answer

Stick with steam itself, they have regular sales, usually with quite significant discounts.

As for hard copies of games that are on steam you will have to wiat for sales in the stores.

Even though Steam does host regular seasonal sales like Chewiesbro said, you should also check out http://cheapsteamgames.com./

I found two great links to lesser-known online stores that sell Steam Gifts and keys for unbelievably cheap through that site without having to wait for the upcoming Summer Sale. Definitely check that out if you're like me and don't want to wait for an official seasonal sale from Valve.

https://www.g2a.com/r/tsumiki3737

Its not free but you can get steam games cheaper than on steam, ive tried a few sites and its the cheapest i can find that's legit and actually gives you the games. all you have to do is sign up and buy the game, hell if you don't wanna sign up you don't have too, they even offer pay pal as a option. you can also buy keys off other user for cheaper (which is risky) but you never know if they will give you what they state but can pay a small amount for someone to watch over what happens and they wont pay them the money if they don't give you a working key or trade the gift.

hope this was of some help! and if you have any questions feel free to ask or message