Sketchfab (https://sketchfab.com/store/3d-models) and itch.io (https://itch.io/game-assets) both have healthy asset marketplaces as well. Unreal also has a decent asset store and Epic has released high quality assets for free (https://www.unrealengine.com/en-US/paragon, https://www.unrealengine.com/marketplace/open-world-demo-col..., https://www.unrealengine.com/marketplace/infiltrator-demo) and Unity has done the same for some of their high-end demos. Unity asset store also has small prototyping kits for cheap. I like https://www.oryxdesignlab.com tilesets for small roguelikes and RPGs as well.