OpenGameArt Discord Channel: https://discord.gg/yDaQ4NcCuxt
Open Game Art (OGA) is a web site and community where users upload art for games that anyone can use in commercial projects, as long as you adhere to the according licence terms.
You can find anything from sound effects to 3D models and concept art. The site is a great resource for finding placeholder art for prototypes and game jam projects. It is very easy to find and read the licenses of the art you can find there.