If you've gone through the Godot doc you've probably seen this already, but just in case, Godot does have an Asset Library of user contributed assets (code and artwork) that is accessible via web browser and (more conveniently) within the Godot editor, so it is similar to the integrated Unity and Unreal asset stores, the big difference being that everything in the Godot library is distributed under an open source license.
https://docs.godotengine.org/en/stable/tutorials/assetlib/what_is_assetlib.html