I also only recently discovered the channel, but oh boy, it's gold! Wish it was around when I started out.
Getting to your question, you should check out the Unity Asset Store (https://www.assetstore.unity3d.com). Lots of free and paid 2D/3D art for commercial use. You can also try Open Game Art (https://opengameart.org/)
One thing to keep in mind though, it's hard to get art that matches. Either by style, quality or detail. I would recommend you find something you like, then check out what else that specific author has on offer. You can usually go with different authors, for different components. Like one for enemies, one for worlds, one for weapons etc. but I wouldn't recommend using too many different sources. All this is considering that you want to sell the game. A much more lighthearted approach will suffice if your intent is only to learn for now.
I would however recommend that you try and make the art yourself. Using the art that is available feels very limiting, there's never exactly what your looking for, and even if there is, there's usually not enough of it. For most of my games I've used art from the Asset Store, but I cannot tell you how much it stifles creativity. IMO there's nothing wrong with using art like this, but you will find that your game/initial vision will suffer. You can work around using only squares, it's much harder however to work around what someone else, unknowingly to you and your vision has already laid out.
All of the best and never lose hope ;)
[EDIT] Changed links to open in new window :)