As someone else mentioned, the Unity asset store has plenty of assets too - some are free, and many others are reasonably priced. There are plenty of 2D assets available, as well as 3D if that's what you're looking for.
Last time I checked, the TOS/EULA says you can use Unity purchased assets outside of Unity games, too - as long as you download them through the Unity editor. So you could potentially use the art assets you buy there even if you want to write your own game engine from scratch.
You might be able to find what you need here, too:
I've been to several game jams where I see amazing prototypes come up in 24-48 hours where people use free or really cheap $5-$40 asset store assets
https://www.google.com/search?q=unity+assets+for+prototyping
It's arguably one reason why there are a glut of games. Not only do engines like Unity and Unreal make it easy to get started but their asset stores have starter kits for almost any type of game. FPS, 3rd person platformer, racing, tower defense game, 2D platformer, etc there's a kit that you download and just start skinning.
https://assetstore.unity.com/search?q=category%3A90&sc=templ...