Have you tried https://godotmarketplace.com/ ?
If you are new to development, then, I think assets are the least to worry about right now. If it's 3D you would like to tackle, perhaps you can use placeholder objects such as different sized cubes or cylinders to move around and to interact with. For more visual fidelity, there are so many free 3D assets out there. Collecting separate assets will provide a better look for your game but their art style may differ so you may end up doing your own or hiring help later in the project.
It's usually easier to find proper 2D assets. One great source was mentioned above. Also, since you didn't mention that it should be free, there is also https://www.gamedevmarket.net/