Best place to buy PS4 games on good deals?

Depends on your country. If you are in the US, I think you can get used PS4 games for like $10 from GameStop

In my experience, Gamefly actually has some pretty good sales usually for used games. The discs and case are in great condition and any codes that came with the game are also unused. Here is a shot from their current sale, be sure to subscribe for email if you want to

there's new thing in ps-store
https://twitter.com/PlayStationEU/status/1019580549491351552
$19.99 for 2years old games xD

From your older brother who still has boxes stored in your parents' basement.

PSN is normally the best place to get cheap games. Sales differ between regions, so a US account and UK account will offer the greatest number of games.

Humble Bundle has done a few PSN bundles, so you might be able to find a trader with a key or two from those. They will require a US account.