https://isthereanydeal.com/ is great for getting any given game at a good price. You can see where it is currently/usually cheapest, its price over time, etc. You can also set up notifications for when it drops below a certain threshold.
In a similar vein, there is also https://www.dekudeals.com/ for the Nintendo switch. It has similar functionality where you can make an account and be notified when games go on sale for consoles.
Also https://gg.deals in the same veins, it list both official stores prices, and key resellers, but with resellers it also lists the risks known for each reseller (like "unknown key sources", "adds a fee after checkout" etc)